P0300
- Powertrain
- Generic (SAE)
- Emissions
Get it looked at now
A flashing check engine light means raw fuel is reaching the catalytic converter fast enough to overheat and melt it, sometimes within minutes of driving. Even with a steady light, a random misfire means unstable combustion, lost power, and possible stalling in traffic.
- Safe to drive
- If the light is flashing, stop driving and get it towed. Steady light and smooth running: drive gently, straight to diagnosis.
Random/Multiple Cylinder Misfire Detected
The computer detected cylinders failing to burn their fuel properly, spread across more than one cylinder rather than stuck on one. That points at something all the cylinders share — spark, fuel supply, air, or timing — not one bad part.
The misfire monitor measures crankshaft acceleration after each combustion event using the crank position sensor, and attributes any missing acceleration to the cylinder that should have produced it. The PCM keeps a misfire counter per cylinder. When the counts are spread across cylinders, or no single cylinder dominates, it sets P0300 rather than, or alongside, a cylinder-specific P030x. Two rate thresholds exist: a catalyst-damaging rate evaluated over roughly a 200-revolution window, which flashes the MIL, and an emissions rate evaluated over roughly 1,000 revolutions, which sets a steady MIL.
How the car detected itContinuous monitor, active whenever the engine runs within enable conditions. Rough-road detection and other filters suppress false counts from driveline vibration. Because the monitor infers combustion from crank speed, it can be misled by a damaged crank reluctor ring, a worn crank sensor, or a slipping torque converter, and some vehicles require a crankshaft variation relearn after certain repairs. Mode 06 data exposes the per-cylinder misfire counts, which is the fastest way to see whether the misfires are genuinely random or clustered on two cylinders that share a coil pack, an intake runner, or one bank.
A trouble code records what a control module measured. It does not identify which part failed. Test before replacing anything.
- Safe to drive
- If the light is flashing, stop driving and get it towed. Steady light and smooth running: drive gently, straight to diagnosis.
- Urgency
- Get it looked at now
- Will it pass emissions
- No
- If ignored
- A sustained misfire dumps unburned fuel into the exhaust. The converter can overheat and melt internally, turning a $300 tune-up into a converter replacement. Unburned fuel also washes oil off the cylinder walls and dilutes the engine oil, accelerating bearing and bore wear. A worsening misfire can leave you stranded.
- Check engine light
- Solid
- Clearing the light
- A completed drive cycle is normally required before the monitor re-runs and the light can clear.
- Check engine light on or flashing
- Rough idle and uneven running
- Power loss, hesitation under acceleration
- Increased fuel consumption
- Possible smell of unburned fuel from exhaust
Many vehicles show no symptom at all beyond the warning light. The absence of a symptom does not mean the fault is not real.
These are the causes this code can have. For the order to work through them in, see what to check first.
| # | Cause | How often | Component |
|---|---|---|---|
| 1 | Worn spark plugs | Very common | — |
| 2 | Vacuum leak affecting all cylinders | Very common | — |
| 3 | Fuel pressure low | Common | — |
| 4 | Timing chain stretched (high mileage) | Occasional | — |
Ordering reflects how often each cause is responsible in general, not a probability for your vehicle. Confirm by testing.
In order. Each of these is cheaper or faster than what follows it, and each one can make the rest unnecessary.
Whether the check engine light is flashing or steady
Flashing means a catalyst-damaging misfire rate right now. That changes the answer from 'book an appointment' to 'stop driving and tow it'.
Per-cylinder misfire counts in Mode 06 or scan-tool live data
It shows whether the misfires are truly random or concentrated. Counts clustered on two cylinders that share a coil pack, or on one bank, is a completely different lead from evenly spread counts.
Every other code stored at the same time
P0300 with lean codes points to a vacuum leak or fuel delivery. P0300 with a cam or crank correlation code points to timing. P0300 with a coolant temperature code points at a cold-running engine. The company a code keeps is most of the diagnosis.
Fuel level and whether symptoms started right after a fill-up
Contaminated fuel or water in the tank produces exactly this pattern and often starts within a few miles of refueling.
Spark plug and ignition service history
Plugs at or past their interval are a common shared cause. Copper plugs typically run around 30,000 miles and iridium or platinum often 60,000 to 100,000, but the interval is vehicle-specific.
Record freeze frame and per-cylinder misfire counts before clearing anything
Tools Scan tool with Mode 06 and freeze frame support
Freeze frame tells you the RPM, load, coolant temperature and fuel trims when the misfire crossed threshold. Mode 06 gives the count distribution. Clearing codes destroys both, and they are the two most useful pieces of evidence on this code.
ExpectedA count pattern you can characterize: evenly spread, clustered on cylinders sharing an ignition component, or clustered on one bank.
Inspect the spark plugs on all cylinders
Tools Spark plug socket, gap gauge, torque wrench
Pull every plug, keep them in cylinder order, and read them side by side. Check gap, electrode wear, deposits and color. The comparison between cylinders is far more informative than any single plug. Oil fouling, coolant-white deposits, or one plug washed clean each point somewhere specific.
ExpectedEven wear and color across all cylinders with gap at specification. Any outlier redirects the diagnosis to that cylinder.
Smoke test for vacuum and intake leaks
Tools Smoke machine
A large shared leak leans out every cylinder, and misfires most obviously at idle where the leak is the largest fraction of total airflow. Check the intake boot, manifold gasket, brake booster hose, PCV path and injector seals. A booster leak large enough to misfire the engine also degrades power brake assist, so treat a hard pedal as a stop-driving symptom rather than a side note.
ExpectedNo smoke escaping. Combined with high positive fuel trims at idle, a leak found here is very likely the cause.
Verify fuel pressure and volume under load
Tools Fuel pressure gauge, or scan tool fuel pressure PID
Relieve fuel system pressure before opening any fuel connection, keep ignition sources away and an extinguisher within reach. Measure at idle and during a snap throttle or loaded test. A supply that holds pressure at idle and collapses under demand starves every cylinder at once, which is exactly the random pattern this code describes. On direct-injection engines read the pressure PIDs with a scan tool rather than opening the high-pressure side.
ExpectedPressure holds specification under load with only a small drop. Values are vehicle-specific.
Test ignition output cylinder by cylinder
Tools Inductive spark tester or lab scope with current clamp
With plugs known good, check spark energy on each cylinder using an inductive pickup, an in-line spark tester, or scan-tool coil current data. On coil-on-plug engines, swap a suspect coil to a different cylinder, clear counts, and see whether the misfire follows it. On wasted-spark systems, note which cylinder pairs share a coil.
ExpectedConsistent, strong spark on every cylinder. A misfire count that moves with a swapped coil identifies that coil.
Run a relative compression or cylinder balance test
Tools Compression gauge, leak-down tester, or lab scope with amp clamp
Relative compression from starter current draw, a running compression test, or a scan-tool power balance test finds mechanical weakness quickly. Follow any low cylinder with a cranking compression test, then a leak-down test to find where the pressure escapes — hiss at the intake means an intake valve, at the exhaust means an exhaust valve, into the crankcase means rings, bubbles in the coolant mean the head gasket.
ExpectedCylinders within roughly 10 to 15% of each other. The absolute compression figure is vehicle-specific.
Check valve timing on higher-mileage engines
Tools Scan tool with cam/crank correlation data, service information
A stretched timing chain retards valve timing on every cylinder and produces exactly this random misfire pattern, often with a rattle for a second or two on cold start. Verify cam-to-crank correlation with a scan tool or by inspection.
ExpectedCam and crank correlation within specification, no correlation codes, no chain rattle on cold start.
Repair, clear counters, and confirm under the original conditions
Tools Scan tool with Mode 06
After repair, clear codes and misfire counters, then drive through the RPM and load conditions recorded in the freeze frame. Recheck Mode 06. Absence of the light alone is not confirmation.
ExpectedMisfire counts at or near zero across the operating range and the misfire monitor reporting complete.
| Repair | Only after you confirm | Typical cost | Who |
|---|---|---|---|
| Replace spark plugs, full set | Plugs out of gap, worn past service interval, or fouled across multiple cylinders | $120–$600 | DIY |
| Replace failing ignition coils or plug wires | Misfire counts follow a swapped coil, or spark output measurably weak on the affected cylinders | $150–$700 | DIY |
| Repair vacuum or intake manifold leak | Smoke test locates the leak and fuel trims are high positive at idle | $100–$700 | DIY |
| Replace fuel pump or fuel filter | Fuel pressure or volume below specification, particularly collapsing under load | $250–$1,400 | Hard |
| Clean or replace fuel injectors | Injector balance or flow test showing multiple injectors delivering low | $150–$1,200 | Hard |
| Drain and replace contaminated fuel | Water or debris found in a fuel sample, with symptoms starting immediately after a fill-up | $200–$800 | Hard |
| Replace timing chain or correct valve timing | Cam-to-crank correlation out of specification, or measured chain stretch on a high-mileage engine | $1,200–$3,500 | Shop |
| Repair mechanical compression loss — valves, rings, or head gasket | Compression test showing low cylinders and leak-down identifying where the pressure escapes | $1,000–$4,500 | Shop |
A plugs-and-coils job on an inline four is often under $400. The same job on a transverse V6, where the intake manifold has to come off to reach the rear bank, can be double that. Pay the diagnostic fee — $100 to $200 of Mode 06 reading and smoke testing prevents most of the expensive guessing on this code, and mechanical repairs at the far end of the range make guessing costly.
- Replacing every coil and plug before reading per-cylinder counts. Sometimes it works. Often it spends $500 on what turns out to be a vacuum leak.
- Clearing the code 'to see if it comes back', which erases freeze frame and misfire counts — the best evidence available.
- Continuing to drive with a flashing light because the car still moves. The converter is being destroyed while you decide.
- Treating P0300 alongside lean codes as an ignition problem. The misfire is a symptom of the lean condition, not the cause.
- Assuming 'random' means 'unrelated'. Counts clustered on two cylinders that share a wasted-spark coil point straight at that coil.
- Skipping compression testing on a high-mileage engine because the plugs looked acceptable.
- Ignition Coil
- Spark Plug
- Fuel Injector
- Intake Manifold
- Timing Chain
- Definition
- Generic code set (SAE J2012 / ISO 15031-6) via the OBDexopen database, dedicated to the public domain under CC0-1.0.
- Diagnostic guidance
- Written and reviewed in-house. See our editorial policy.
- Review status
- Reviewed
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