P0300code home
Method & sources

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P0300 decision tool

P0300 Find what to check first

Use your code list, symptoms, and recent work. The check order updates instantly.

Code meaning Random / multiple cylinder misfire Rule-based triage · not a scan of your vehicle

Evidence in use

0 of 6 answered

01SafetyWaiting 02CodesWaiting 03ConditionWaiting 04SymptomsWaiting 05ReplacedWaiting 06TriggerWaiting
0 of 6

Each answer re-ranks the same five checks. No submit step.

Is the check-engine light flashing? Use the engine-running event, not the key-on bulb check.

If it flashed while running then went off, select flashing. Serious symptoms?

Check whether the light is flashing. Check the running-event light and symptoms. This tool cannot certify that driving is safe.

Evidence-based check order

What to check next

Based on 0 of 6 answers
SafetyWaiting
CodesWaiting
ConditionWaiting
HistoryWaiting

Check the running-event light and symptoms. A lamp alone cannot establish driving safety.

  1. Pull the spark plugs and read them

    Tools / experienceConfidence: low

    Suspect: worn, fouled, mis-gapped, or wrong spark plugs

    Why this is here

    • None of the entered evidence specifically raises this path. It remains in the order because it is a common P0300 check.

    Checking it rules out

    worn plugsfoulingwrong heat rangecarbon tracking on the insulator

    Doing this yourself

    Needs some experience Engine off and cool. Plugs read like a diagnostic instrument — colour, gap, and tracking each mean something.

    Time
    30–90 minutes
    Cost to check
    $0 if you do it yourself · $60–150 if a shop reads them
    Cost to fix
    $100–400 on a four-cylinder, up to about $700 on a V8 where plugs sit under the intake

    Plugs that come out clean and correctly gapped move this to the bottom and promote fuel and air causes.

    Review before reporting clear
    Before recording this path as reported clear

    Exact check: Pull the spark plugs and read them

    Method / safety boundary: Engine off and cool. Plugs read like a diagnostic instrument — colour, gap, and tracking each mean something.

    Scoped no-fault result from this exact check: No removed plug had a wrong part number or gap, abnormal wear or fouling, cracked ceramic, or tracking in the inspected set.

    Record only when all three statements are known. This is a self-reported session exclusion—not a diagnosis, repair verification, component guarantee, or safety clearance.

  2. Swap two ignition coils and re-scan

    Tools / experienceConfidence: low

    Suspect: a failing ignition coil

    Why this is here

    • None of the entered evidence specifically raises this path. It remains in the order because it is a common P0300 check.

    Checking it rules out

    one weak coila coil breaking down under load

    Doing this yourself

    Needs some experience Engine off. Use a cylinder-specific code or misfire counter to choose the suspect coil, and preserve codes and freeze-frame data before any reset. Repeat the original condition only when the safety result allows it.

    Time
    20–40 minutes plus a safe, vehicle-appropriate recheck
    Cost to check
    $0 — the controlled swap test costs nothing but time
    Cost to fix
    $60–350 per coil depending on the engine and whether it is coil-on-plug

    If the same cylinder remains the leader after the coil moves, that coil moves down; if the misfire follows, the coil path moves up.

    Review before reporting clear
    Before recording this path as reported clear

    Exact check: Swap two ignition coils and re-scan

    Method / safety boundary: Engine off. Use a cylinder-specific code or misfire counter to choose the suspect coil, and preserve codes and freeze-frame data before any reset. Repeat the original condition only when the safety result allows it.

    Scoped no-fault result from this exact check: In a controlled cylinder-to-cylinder swap, the misfire did not follow the moved coil under a safe comparable recheck.

    Record only when all three statements are known. This is a self-reported session exclusion—not a diagnosis, repair verification, component guarantee, or safety clearance.

  3. Smoke-test the intake for unmetered air

    Shop testConfidence: low

    Suspect: a vacuum, intake-gasket, or PCV leak

    Why this is here

    • None of the entered evidence specifically raises this path. It remains in the order because it is a common P0300 check.

    Checking it rules out

    cracked vacuum hosesintake manifold gasket leakPCV path leaka hose left off during earlier work

    Doing this yourself

    Shop work — not a driveway job Shop test. It needs a smoke machine. Spraying carburettor cleaner or propane around a hot running engine is an ignition source, and this tool does not recommend it.

    Time
    20–40 minutes at a shop
    Cost to check
    $0 when a shop bundles it into diagnosis · $40–120 standalone
    Cost to fix
    $10–60 for a hose · $150–450 for an intake gasket, more where the manifold is buried

    A fuel trim that stays high at cruise instead of falling moves this down and fuel delivery up.

    Review shop-reported result
    Before recording this path as reported clear

    Exact check: Smoke-test the intake for unmetered air

    Method / safety boundary: Shop test. It needs a smoke machine. Spraying carburettor cleaner or propane around a hot running engine is an ignition source, and this tool does not recommend it.

    Scoped no-fault result from this exact check: A smoke test found no leak in the intake, vacuum-hose, or PCV paths it covered.

    Record only when all three statements are known. This is a self-reported session exclusion—not a diagnosis, repair verification, component guarantee, or safety clearance.

  4. Test fuel pressure running and on leak-down

    Shop testConfidence: low

    Suspect: low or falling fuel pressure — pump, filter, or regulator

    Why this is here

    • None of the entered evidence specifically raises this path. It remains in the order because it is a common P0300 check.

    Checking it rules out

    weak fuel pumprestricted filterleaking regulatora check valve that will not hold

    Doing this yourself

    Shop work — not a driveway job Shop test. Fuel rails hold pressure after shutdown and fuel spray near a hot engine is a fire risk.

    Time
    30–60 minutes
    Cost to check
    $60–150, usually inside a diagnostic charge
    Cost to fix
    $60–200 for a filter · $400–1,300 for an in-tank pump, driven mostly by labour

    Pressure that matches the vehicle specification running and on leak-down moves the shared fuel-supply path down; it does not exclude an individual injector fault.

    Review shop-reported result
    Before recording this path as reported clear

    Exact check: Test fuel pressure running and on leak-down

    Method / safety boundary: Shop test. Fuel rails hold pressure after shutdown and fuel spray near a hot engine is a fire risk.

    Scoped no-fault result from this exact check: Running and leak-down fuel pressure matched the vehicle specification; this does not exclude an individual injector.

    Record only when all three statements are known. This is a self-reported session exclusion—not a diagnosis, repair verification, component guarantee, or safety clearance.

  5. Compare injector behaviour cylinder to cylinder

    Shop testConfidence: low

    Suspect: a dirty, leaking, or electrically failing injector

    Why this is here

    • None of the entered evidence specifically raises this path. It remains in the order because it is a common P0300 check.

    Checking it rules out

    a clogged injectora leaking injectoran open injector circuit

    Doing this yourself

    Shop work — not a driveway job Shop test. A listening check can show only that an injector clicks; clearing the full path requires electrical or balance/flow evidence, and anything that opens the fuel rail carries fire risk.

    Time
    30–60 minutes
    Cost to check
    $80–200 for an electrical, balance, or drop test
    Cost to fix
    $150–400 for cleaning · $200–600 per injector fitted, far more on direct-injection engines

    Even injector behaviour across cylinders moves this down.

    Review shop-reported result
    Before recording this path as reported clear

    Exact check: Compare injector behaviour cylinder to cylinder

    Method / safety boundary: Shop test. A listening check can show only that an injector clicks; clearing the full path requires electrical or balance/flow evidence, and anything that opens the fuel rail carries fire risk.

    Scoped no-fault result from this exact check: Electrical or balance/flow comparison found no uneven, leaking, restricted, or open injector within scope.

    Record only when all three statements are known. This is a self-reported session exclusion—not a diagnosis, repair verification, component guarantee, or safety clearance.

Visual evidence Diagnostic path map · add scanner data for two more charts

Exact scanner evidence

See the evidence without fake probabilities

Every mark below is an exact check state or a value you entered. Nothing is a likelihood score.

Diagnostic path mapExact current order — not likelihood or probability5 active · 11 later · 0 reported clear

Open a path for its test or Undo. Order is not certainty.

Active next checks

5

Later paths

11
Fuel-trim profile Plot idle, cruise, and optional bank-2 LTFT on the same fixed scale.
Misfire-count distribution Plot only the cylinders and exact counts you enter from Mode $06 or live data.
Less likely, but possible Everything the ranking did not put in the top five

  1. Inspect plug boots and secondary wires for tracking

    Tools / experienceConfidence: low

    Suspect: carbon tracking, split boots, or failing plug wires

    Why this is here

    • None of the entered evidence specifically raises this path. It remains in the order because it is a common P0300 check.

    Checking it rules out

    arcing to groundcarbon-tracked bootsa boot that never seated

    Doing this yourself

    Needs some experience Engine off. Look for grey or black tracking lines on the insulator and inside the boot.

    Time
    20–40 minutes
    Cost to check
    $0 while the plugs are out
    Cost to fix
    $20–120 for boots · $60–200 for a wire set

    Clean, supple boots with no tracking move this path down.

    Review before reporting clear
    Before recording this path as reported clear

    Exact check: Inspect plug boots and secondary wires for tracking

    Method / safety boundary: Engine off. Look for grey or black tracking lines on the insulator and inside the boot.

    Scoped no-fault result from this exact check: No inspected boot or secondary wire had tracking, splits, poor seating, or visible arcing evidence.

    Record only when all three statements are known. This is a self-reported session exclusion—not a diagnosis, repair verification, component guarantee, or safety clearance.

  2. Compare the air-metering reading against calculated load

    Tools / experienceConfidence: low

    Suspect: a MAF or MAP sensor reporting the wrong airflow

    Why this is here

    • None of the entered evidence specifically raises this path. It remains in the order because it is a common P0300 check.

    Checking it rules out

    a dirty or drifting MAFa MAP sensor reading out of rangea leak between the MAF and the throttle

    Doing this yourself

    Needs some experience Scan-tool work with the engine running. Keep hands and clothing clear of belts and fans.

    Time
    20–40 minutes
    Cost to check
    $0 with a scan tool · $60–150 at a shop
    Cost to fix
    $15 for cleaner · $90–400 for a sensor

    Fuel trims that are normal at cruise make an airflow-reporting fault unlikely.

    Review before reporting clear
    Before recording this path as reported clear

    Exact check: Compare the air-metering reading against calculated load

    Method / safety boundary: Scan-tool work with the engine running. Keep hands and clothing clear of belts and fans.

    Scoped no-fault result from this exact check: MAF or MAP values matched calculated load and vehicle-specific expectations under the tested conditions.

    Record only when all three statements are known. This is a self-reported session exclusion—not a diagnosis, repair verification, component guarantee, or safety clearance.

  3. Run a compression or leak-down test

    Shop testConfidence: low

    Suspect: lost compression — valves, rings, head gasket, or a collapsed lifter

    Why this is here

    • None of the entered evidence specifically raises this path. It remains in the order because it is a common P0300 check.

    Checking it rules out

    a burnt valveworn ringsa head gasket between cylindersa collapsed cylinder-deactivation lifter

    Doing this yourself

    Shop work — not a driveway job Shop test. It runs with the ignition disabled and the throttle held open, and a leak-down test uses shop air.

    Time
    1–2 hours
    Cost to check
    $100–250
    Cost to fix
    $400–1,200 for a head gasket on a simple engine · $2,000–5,000 and up for internal repair

    Closely matched readings move a major compression imbalance down, but do not prove every intermittent mechanical fault absent.

    Review shop-reported result
    Before recording this path as reported clear

    Exact check: Run a compression or leak-down test

    Method / safety boundary: Shop test. It runs with the ignition disabled and the throttle held open, and a leak-down test uses shop air.

    Scoped no-fault result from this exact check: Compression or leak-down was closely matched with no major leak path; intermittent mechanical faults remain possible.

    Record only when all three statements are known. This is a self-reported session exclusion—not a diagnosis, repair verification, component guarantee, or safety clearance.

  4. Check cam-to-crank correlation and chain condition

    Shop testConfidence: low

    Suspect: timing chain stretch, a slipped belt, or a crank/cam sensor fault

    Why this is here

    • None of the entered evidence specifically raises this path. It remains in the order because it is a common P0300 check.

    Checking it rules out

    chain stretcha tooth out on reassemblya failing crank or cam sensor

    Doing this yourself

    Shop work — not a driveway job Shop work. Correlation is read with a scan tool; confirming it means opening the front of the engine.

    Time
    1–3 hours to diagnose
    Cost to check
    $100–250
    Cost to fix
    $200–500 for a sensor · $900–3,000 for a chain and guides, dominated by labour

    A correlation reading inside specification moves this to the bottom.

    Review shop-reported result
    Before recording this path as reported clear

    Exact check: Check cam-to-crank correlation and chain condition

    Method / safety boundary: Shop work. Correlation is read with a scan tool; confirming it means opening the front of the engine.

    Scoped no-fault result from this exact check: Cam-to-crank correlation and physical timing checks were inside the vehicle-specific specification.

    Record only when all three statements are known. This is a self-reported session exclusion—not a diagnosis, repair verification, component guarantee, or safety clearance.

  5. Review the last fuel fill and fuel quality

    Basic checkConfidence: low

    Suspect: contaminated fuel, water, or the wrong octane

    Why this is here

    • None of the entered evidence specifically raises this path. It remains in the order because it is a common P0300 check.

    Checking it rules out

    bad fuelwater in the tankwrong octane for the engine

    Doing this yourself

    Anyone can do this Only continue using the tank when the page’s safety result allows normal driving. Note the station and fill; if driving is limited or unsafe, a shop should sample or drain the fuel instead.

    Time
    One safe fuel-use cycle, or a shop sample/drain
    Cost to check
    $0
    Cost to fix
    $0 if it clears · $200–600 to drain and purge a badly contaminated tank

    A misfire that clearly started before the last fill moves fuel quality down.

    Review before reporting clear
    Before recording this path as reported clear

    Exact check: Review the last fuel fill and fuel quality

    Method / safety boundary: Only continue using the tank when the page’s safety result allows normal driving. Note the station and fill; if driving is limited or unsafe, a shop should sample or drain the fuel instead.

    Scoped no-fault result from this exact check: The misfire predates the last fill and the fuel review found no link to that fill or sample.

    Record only when all three statements are known. This is a self-reported session exclusion—not a diagnosis, repair verification, component guarantee, or safety clearance.

  6. Check the EGR valve for flow when it should be closed

    Tools / experienceConfidence: low

    Suspect: an EGR valve stuck open or leaking at idle

    Why this is here

    • None of the entered evidence specifically raises this path. It remains in the order because it is a common P0300 check.

    Checking it rules out

    a stuck EGR valvea carboned EGR port

    Doing this yourself

    Needs some experience Engine off for inspection. Exhaust-side components stay hot long after shutdown.

    Time
    30–60 minutes
    Cost to check
    $0 for a visual and manual check · $60–150 at a shop
    Cost to fix
    $150–500 for a valve · $200–700 to clean carboned ports

    A misfire that appears only under load and not at idle makes this unlikely.

    Review before reporting clear
    Before recording this path as reported clear

    Exact check: Check the EGR valve for flow when it should be closed

    Method / safety boundary: Engine off for inspection. Exhaust-side components stay hot long after shutdown.

    Scoped no-fault result from this exact check: The EGR valve stayed closed with no unintended flow when commanded closed.

    Record only when all three statements are known. This is a self-reported session exclusion—not a diagnosis, repair verification, component guarantee, or safety clearance.

  7. Inspect ignition and injector connectors and engine grounds

    Tools / experienceConfidence: low

    Suspect: a chafed harness, corroded connector, or poor engine ground

    Why this is here

    • None of the entered evidence specifically raises this path. It remains in the order because it is a common P0300 check.

    Checking it rules out

    a broken or backed-out connector pina corroded ground straprodent damage

    Doing this yourself

    Needs some experience Engine off. Wiggle-test connectors and check that ground straps are clean and tight.

    Time
    30–60 minutes
    Cost to check
    $0
    Cost to fix
    $0–150 for a connector or ground repair · more if a harness section is damaged

    A misfire that is completely steady rather than intermittent makes a connection fault less likely.

    Review before reporting clear
    Before recording this path as reported clear

    Exact check: Inspect ignition and injector connectors and engine grounds

    Method / safety boundary: Engine off. Wiggle-test connectors and check that ground straps are clean and tight.

    Scoped no-fault result from this exact check: No inspected connector, harness section, or engine ground showed a fault under the checks performed.

    Record only when all three statements are known. This is a self-reported session exclusion—not a diagnosis, repair verification, component guarantee, or safety clearance.

  8. Check whether the EVAP purge valve is stuck open

    Tools / experienceConfidence: low

    Suspect: a purge valve flooding the intake with fuel vapour at idle

    Why this is here

    • None of the entered evidence specifically raises this path. It remains in the order because it is a common P0300 check.

    Checking it rules out

    a stuck-open purge solenoid

    Doing this yourself

    Needs some experience Engine off. A purge valve should not flow with no vacuum applied and no command.

    Time
    20–40 minutes
    Cost to check
    $0 for a bench check
    Cost to fix
    $40–200 for the valve

    No EVAP code and a clean idle after a fresh fill make this unlikely.

    Review before reporting clear
    Before recording this path as reported clear

    Exact check: Check whether the EVAP purge valve is stuck open

    Method / safety boundary: Engine off. A purge valve should not flow with no vacuum applied and no command.

    Scoped no-fault result from this exact check: The purge valve passed no flow when uncommanded and responded when commanded.

    Record only when all three statements are known. This is a self-reported session exclusion—not a diagnosis, repair verification, component guarantee, or safety clearance.

  9. Check for exhaust back-pressure

    Shop testConfidence: low

    Suspect: a clogged converter or crushed pipe choking the engine

    Why this is here

    • None of the entered evidence specifically raises this path. It remains in the order because it is a common P0300 check.

    Checking it rules out

    a plugged catalytic convertera collapsed exhaust section

    Doing this yourself

    Shop work — not a driveway job Shop test. It runs with the engine hot and involves working at the exhaust.

    Time
    30–60 minutes
    Cost to check
    $60–150
    Cost to fix
    $800–2,500 for a converter, driven by vehicle and emissions certification

    A misfire that is worst at idle and clears with revs points away from a restriction.

    Review shop-reported result
    Before recording this path as reported clear

    Exact check: Check for exhaust back-pressure

    Method / safety boundary: Shop test. It runs with the engine hot and involves working at the exhaust.

    Scoped no-fault result from this exact check: Exhaust back-pressure remained inside the vehicle specification under the tested condition.

    Record only when all three statements are known. This is a self-reported session exclusion—not a diagnosis, repair verification, component guarantee, or safety clearance.

  10. Re-check the ignition work that was just done

    Tools / experienceConfidence: low

    Suspect: a gap, seating, connector, or hose mistake from the recent job

    Why this is here

    • None of the entered evidence specifically raises this path. It remains in the order because it is a common P0300 check.

    Checking it rules out

    wrong gapwrong plug part numbera coil not fully seateda connector not latcheda vacuum line left disconnected

    Doing this yourself

    Needs some experience Engine off. Re-checking your own work is free and is the highest-yield thing to do when a misfire appears right after an ignition job.

    Time
    30–60 minutes
    Cost to check
    $0
    Cost to fix
    $0 if it is a seating or hose error · the cost of the correct part if the wrong one went in

    Work done long before the misfire started makes this unlikely.

    Review before reporting clear
    Before recording this path as reported clear

    Exact check: Re-check the ignition work that was just done

    Method / safety boundary: Engine off. Re-checking your own work is free and is the highest-yield thing to do when a misfire appears right after an ignition job.

    Scoped no-fault result from this exact check: Recent ignition work had correct parts, gaps, seating, connectors, and hoses with no installation fault found.

    Record only when all three statements are known. This is a self-reported session exclusion—not a diagnosis, repair verification, component guarantee, or safety clearance.

  11. Check the oil level, grade, and PCV path

    Tools / experienceConfidence: low

    Suspect: overfilled or wrong-grade oil, or oil being drawn into the intake

    Why this is here

    • None of the entered evidence specifically raises this path. It remains in the order because it is a common P0300 check.

    Checking it rules out

    overfillwrong viscosity affecting variable valve timinga PCV path pulling oil into the intake

    Doing this yourself

    Needs some experience Engine off and cool. The dipstick and oil specification are basic checks, but clearing the full PCV/oil-ingestion path also requires inspection of accessible hoses and intake evidence; use a shop where access or disassembly is required.

    Time
    10 minutes
    Cost to check
    $0
    Cost to fix
    $0 to drain an overfill · $60–150 for an oil change with the correct grade · $80–300 for PCV parts

    A correct oil level and grade, with no oil found in the intake path, moves this down.

    Review before reporting clear
    Before recording this path as reported clear

    Exact check: Check the oil level, grade, and PCV path

    Method / safety boundary: Engine off and cool. The dipstick and oil specification are basic checks, but clearing the full PCV/oil-ingestion path also requires inspection of accessible hoses and intake evidence; use a shop where access or disassembly is required.

    Scoped no-fault result from this exact check: Oil level and grade were correct, and inspected PCV/intake paths showed no oil-ingestion evidence.

    Record only when all three statements are known. This is a self-reported session exclusion—not a diagnosis, repair verification, component guarantee, or safety clearance.

Continue the triage

Add the evidence you have

Choose what you know. “Not sure” is an answer, not “nothing wrong”. Six replies do not certify a diagnosis.

Find the right path for your situation
Worried about driving?

Start with the running-event light. Stalling, severe power loss, fuel smell or red oil/temperature warnings take priority over any check order.

No scanner or incomplete printout

Choose unknown rather than claiming no other codes. Ask for the original codes and freeze frame before clearing diagnostic memory.

Already replaced parts

Record only known work. A new part is not proof of a fault-free installation; the result asks for tests, not another shopping list.

Have a report or checking after repair?

Establish the original evidence first. Then open the full report to use scanner charts, the test request or Run verification. A clean run is not permanent repair certification.

Need a cost or mechanic handoff?

Open the full diagnostic report for check/fix ranges and Copy summary. Ranges are indicative, not a quote; request measured findings and an itemized estimate.

Vehicle-specific limits: this general spark-ignition P0300 workflow does not supply VIN-specific service procedures, cylinder layout or recall applicability. Diesel, alternative-fuel and non-engine faults need the correct vehicle procedure. No remote result certifies driving safety or emissions readiness.

What else was on the scan printout? The companion-code set changes the check order more than make, model, or mileage.

Start from a common code pattern

Applies only the code list. Change any answer afterwards.

Spaces, commas and new lines all work. PO300 with a letter O is corrected to P0300.

No scanner? Keep unknowns, then request the original code list and freeze frame. This page cannot confirm P0300 from symptoms.

Cylinder-specific misfire codes

Other codes that change the answer

When does it act up? Each condition points at a different set of causes.
What can you feel, hear, or smell? Select everything that applies.
What has already been replaced? Previous replacements change the check order. They do not prove the new part or its installation is fault-free.
Did it start right after something? Timing is a clue, not proof that the work or fill-up caused the misfire.
View your current check order The ranking is already updated — there is no submit step.
Method, limits & sources See the rules behind the ranking, what the tool cannot know, and the dated sources.

How this decides, and what it cannot know

What P0300 actually says

P0300 is the generic OBD-II code for a random or multiple cylinder misfire. It reports that the engine computer detected a misfire pattern without providing a reliable specific- cylinder answer in the generic code list.

The regulation generally requires cylinder identification, but it also allows a general misfire code when the cylinder cannot be identified reliably, and a multiple-cylinder code does not have to list each participating cylinder. So P0300 alone does not prove that the misfire is evenly distributed or that a single-cylinder fault is absent.

Flashing versus steady

The regulation requires the light to blink or flash once per second while catalyst-damaging misfire is occurring, and that fault sets on a single trip rather than waiting for a second one.

So a flashing light is not a more urgent version of the same message — it is a different message. Keep driving and unburned fuel keeps reaching the converter.

You may read elsewhere that “if P0300 is your only code the light will not be flashing.” That is wrong. P0300 can be present during catalyst-damaging misfire and a flashing MIL even when the generic code list contains no P030x cylinder code.

Which cylinder is it?

P0300 does not localise the event by itself. The honest answer is a procedure, not a part: read manufacturer-enhanced or Mode $06 misfire counters, then use a safe, controlled test to separate ignition, fuel and compression on the cylinders the evidence actually names.

Cylinder numbering is not universal across manufacturers, and waste-spark systems fire in companion pairs, so a single failing coil can misfire two cylinders that are nowhere near each other. This tool never tells you where a numbered cylinder physically sits.

If you also have P0420

Fix the misfire first. Raw fuel from a misfire is what destroys a converter, so a converter replaced while the misfire is still there is on the same path as the one it replaced.

How the ranking works

Each answer fires named rules that move candidates up or down. Nothing is random and nothing is weighted by what a part costs. The same answers always produce the same order, and every row shows the rules that put it there.

Confidence has a definition rather than a feeling:

  • High — something you told us points specifically here, and nothing argues against it.
  • Moderate — your answers support it, but nothing separates it decisively.
  • Low — nothing in your answers points here; it is listed because it is a common cause.

What this tool does not know

  • It has not scanned your vehicle. Inputs are owner reports or values you explicitly applied; unknowns are not confirmed normal results.
  • It does not know your engine's cylinder layout, wiring, or which coil serves which cylinder.
  • It cannot see fuel pressure, compression, or live data unless you enter them.
  • Costs are ranges, and real prices vary with vehicle, region, and labour rate.
  • It narrows where to look. It does not replace hands-on testing, and a flashing light means stop driving regardless of what it says.

Sources

Running-event versus pre-start lamp behavior: Ford owner-manual warning lamps. Follow your own vehicle manual; this tool uses conservative stop guidance, not remote driving certification.

Core sources checked 21 August 2026. Scenario safety reviewed 5 September 2026.

Copy a link to this result

The link contains your answers, checked-clear session, scanner values, the current verification, and up to three timeline runs. Anyone who opens it sees the same browser-only result. Losing the link loses the timeline.

They are encoded after the #, which browsers do not send in the request — so the answers still never reach a server.

Privacy

Last updated 24 August 2026.

Everything this tool does happens in your browser. Your codes, symptoms, repairs, scan data, checked-clear session, current verification, and up to three timeline runs are not sent to a diagnostic service, an analytics provider, an error reporter, or an account system. There is no server here to send them to.

No cookies are set, and your answers are not written to local storage or a database. Ordinary web-server security records can include the requested path, time, IP address and user agent — your answers are not part of any request.

When you deliberately copy a result link, your answers are encoded in the URL fragment after #. Browsers do not include that fragment in the HTTP request, but anyone you send the link to can read the answers from it.

Links to the regulation and to trade publications leave this site and are governed by those publishers.

Terms

Last updated 24 August 2026.

This is a free diagnostic aid. It narrows where to look based on what you tell it. It is not a scan of your vehicle, not a repair authorisation, not an emissions-readiness certificate, and not a substitute for hands-on testing by a qualified technician. Each verification result describes one reported retest only; a timeline preserves up to three such reports but does not certify that a repair is permanent or predict whether P0300 will return.

Vehicle repair involves real hazards: pressurised fuel, hot exhaust, high-voltage ignition, moving parts and raised vehicles. Do not attempt a check the tool marks as shop work. If the check-engine light is flashing, stop driving regardless of anything shown here.

Cost figures are wide ranges for orientation only. Actual prices vary with vehicle, parts source, region and labour rate.

Reference thresholds and the regulatory rules cited are dated and sourced. They are general engineering references, not manufacturer service information for your specific vehicle.

The service is provided as available, without warranty.