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OBD-II diagnostic trouble code

P0300 — random / multiple cylinder misfire

Your engine is misfiring and the computer could not pin it to one cylinder. Answer what you know and this ranks what to check first — and shows the rule behind every position.

1 Is the check-engine light flashing or steady? This is the one answer that decides whether it is safe to keep driving.

Flashing means it blinks roughly once a second while you drive. Steady means it stays on without blinking.

Check whether the light is flashing. A flashing light and a steady light call for completely different actions. It is the first thing worth knowing.

What to check first

Ranked by your answers

Based on 0 of 6 answers

Is the check-engine light flashing or steady? This is the one answer that changes whether it is safe to drive.

  1. Pull the spark plugs and read them

    Confidence: low

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

    Why this is here

    • Nothing in your answers points here specifically. It is listed because it is a common cause of this code.

    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.

  2. Swap two ignition coils and re-scan

    Confidence: low

    Suspect: a failing ignition coil

    Why this is here

    • Nothing in your answers points here specifically. It is listed because it is a common cause of this code.

    Checking it rules out

    one weak coila coil breaking down under load

    Doing this yourself

    Needs some experience Engine off. Move a coil to a different cylinder, clear the code, drive the same way, then re-scan. If the misfire follows the coil, you have your answer without buying anything.

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

    A misfire that does not follow the swapped coil eliminates that coil outright.

  3. Smoke-test the intake for unmetered air

    Confidence: low

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

    Why this is here

    • Nothing in your answers points here specifically. It is listed because it is a common cause of this code.

    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.

  4. Test fuel pressure running and on leak-down

    Confidence: low

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

    Why this is here

    • Nothing in your answers points here specifically. It is listed because it is a common cause of this code.

    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 holds at spec both running and on leak-down eliminates the fuel supply.

  5. Compare injector behaviour cylinder to cylinder

    Confidence: low

    Suspect: a dirty, leaking, or electrically failing injector

    Why this is here

    • Nothing in your answers points here specifically. It is listed because it is a common cause of this code.

    Checking it rules out

    a clogged injectora leaking injectoran open injector circuit

    Doing this yourself

    Needs some experience A stethoscope check for a clicking injector is safe with care. Anything that opens the fuel rail is shop work.

    Time
    30–60 minutes
    Cost to check
    $0 for a listening check · $80–200 for a 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.

Less likely, but possible Everything the ranking did not put in the top five

  1. Inspect plug boots and secondary wires for tracking

    Confidence: low

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

    Why this is here

    • Nothing in your answers points here specifically. It is listed because it is a common cause of this code.

    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 rule this out.

  2. Compare the air-metering reading against calculated load

    Confidence: low

    Suspect: a MAF or MAP sensor reporting the wrong airflow

    Why this is here

    • Nothing in your answers points here specifically. It is listed because it is a common cause of this code.

    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.

  3. Run a compression or leak-down test

    Confidence: low

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

    Why this is here

    • Nothing in your answers points here specifically. It is listed because it is a common cause of this code.

    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

    Compression within about 10 percent across all cylinders effectively rules this out.

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

    Confidence: low

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

    Why this is here

    • Nothing in your answers points here specifically. It is listed because it is a common cause of this code.

    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.

  5. Check what went in the tank, then run it out

    Confidence: low

    Suspect: contaminated fuel, water, or the wrong octane

    Why this is here

    • Nothing in your answers points here specifically. It is listed because it is a common cause of this code.

    Checking it rules out

    bad fuelwater in the tankwrong octane for the engine

    Doing this yourself

    Anyone can do this Anyone can do this. Note the station and date, then run the tank low and refill with fuel from somewhere else.

    Time
    One tank
    Cost to check
    $0
    Cost to fix
    $0 if it clears · $200–600 to drain and purge a badly contaminated tank

    A misfire that started well before the last fill rules this out.

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

    Confidence: low

    Suspect: an EGR valve stuck open or leaking at idle

    Why this is here

    • Nothing in your answers points here specifically. It is listed because it is a common cause of this code.

    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.

  7. Inspect ignition and injector connectors and engine grounds

    Confidence: low

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

    Why this is here

    • Nothing in your answers points here specifically. It is listed because it is a common cause of this code.

    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.

  8. Check whether the EVAP purge valve is stuck open

    Confidence: low

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

    Why this is here

    • Nothing in your answers points here specifically. It is listed because it is a common cause of this code.

    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.

  9. Check for exhaust back-pressure

    Confidence: low

    Suspect: a clogged converter or crushed pipe choking the engine

    Why this is here

    • Nothing in your answers points here specifically. It is listed because it is a common cause of this code.

    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.

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

    Confidence: low

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

    Why this is here

    • Nothing in your answers points here specifically. It is listed because it is a common cause of this code.

    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.

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

    Confidence: low

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

    Why this is here

    • Nothing in your answers points here specifically. It is listed because it is a common cause of this code.

    Checking it rules out

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

    Doing this yourself

    Anyone can do this Anyone can do this. Check the dipstick on level ground with the engine off and cool.

    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 in the intake tract, rules this out.

2 When does it act up? Each condition points at a different set of causes.
3 Which other codes are stored? The single most useful thing you can tell this tool. A free scan at most parts stores prints the full list.

Cylinder-specific misfire codes

Other codes that change the answer

Commas, spaces or new lines are all fine. PO300 with a letter O is read as P0300.

4 What can you feel, hear, or smell? Select everything that applies.
5 What has already been replaced? Anything you list is taken off the recommendations — you will not be told to redo it.
6 Did it start right after something? When a misfire appears the same day as a repair or a fill-up, that is usually the cause.

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 is stored when the engine computer sees the crankshaft slow momentarily in the pattern a failed combustion event produces, and cannot attribute those events to one cylinder.

The attribution rule is written into the regulation, which is why the presence or absence of a cylinder code carries real information: a cylinder-specific code is stored when more than 90 % of misfires come from one cylinder, and a multiple-cylinder code is stored when two or more cylinders each account for more than 10 % of them.

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. A catalyst-damaging misfire spread across cylinders stores P0300 and flashes the light.

Which cylinder is it?

P0300 names none, by design. The honest answer is a procedure, not a part: read live misfire counters, then confirm by swapping a coil to another cylinder and seeing whether the misfire follows it.

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. Everything it has is what you typed.
  • 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

Sources checked 16 August 2026.

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Privacy

Last updated 16 August 2026.

Everything this tool does happens in your browser. Your codes, symptoms, repairs and any scan data you type 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.

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

Terms

Last updated 16 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, and not a substitute for hands-on testing by a qualified technician.

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.