P0300: Random or Multiple Cylinder Misfire Detected

What P0300 means

P means powertrain (engine and transmission). The 0 marks a generic code defined by SAE, so the same number means the same thing across makes. The third character, 3, places it in the ignition system or misfire subsystem; the last two digits identify the specific fault.

The engine computer detects misfires by watching tiny changes in crankshaft speed: each cylinder that fires properly gives the crank a push, and one that does not shows up as a momentary slowdown. It counts misfires per cylinder and sets a code when the rate is high enough to raise tailpipe pollution or risk the catalytic converter.

What “Misfire Detected” means here

The misfire monitor has counted misfires above its threshold but they are not confined to one cylinder (or the cylinder was not identified). A cause shared by all cylinders is more likely: vacuum leaks, low fuel pressure, fuelling errors, contaminated fuel, or ignition problems common to several cylinders.

What you may notice

Symptoms depend on the vehicle and on how the module responds to the fault. Any of these can accompany P0300:

• Shaking or rough running, often worst at idle or under load
• A flashing check engine light during severe misfiring
• Loss of power and hesitation
• Fuel smell from the exhaust

Causes to consider

• Worn spark plugs
• Failing ignition coils
• Low fuel pressure
• Vacuum leak

Where to start diagnosing P0300

1. Start with ignition: inspect the spark plug and coil for the cylinder concerned, and swap the coil with a neighbouring cylinder to see if the misfire moves.
2. Then fuel: check the injector connector and resistance, and swap injectors if practical.
3. Then mechanical: a compression or leak-down test identifies valve, ring or head gasket problems.
4. Check for vacuum leaks near the affected cylinder's intake runner.

Clear the code only after recording freeze frame data: it shows the engine speed, load and temperature at the moment the fault set, which is often the best clue to reproducing it.

Live data to watch

These standard OBD2 values relate to P0300. Support varies by vehicle; a value your car does not report will simply be missing.

Related codes

Codes from the same misfire monitor family. If several are stored together, the shared part — wiring, a common reference voltage, or the component itself — is the place to start.

Further reading

• It failed the emissions test
• How engine computers use knock sensors
• OBD2 readiness monitors explained

Symptoms that lead to P0300

If you have not scanned the car yet, or P0300 is one of several codes stored, these guides start from what the vehicle is actually doing.

• Why your engine shakes at idle A car that shudders at a standstill but smooths out once you are moving is almost always losing combustion in one or…
• Something is rattling underneath Most under-car rattles are cheap to fix and loud out of proportion to their seriousness. A small number are not. The…
• It hesitates when you ask for power A flat spot, stumble or brief hole when you open the throttle means the engine is not getting the air, fuel or spark…
• It stalls when you come to a stop Stalling as you lift off and the revs drop to idle means the engine cannot sustain itself at its lowest operating…

Frequently Asked Questions