OBD-II · Engine and powertrain (P)
P0300
Random/multiple cylinder misfire detected
What it means
The ECU detects misfires on more than one cylinder, or cannot pin them to one cylinder. If the warning light flashes, reduce load and stop as soon as it is safe: unburnt fuel can destroy the catalytic converter.
Symptoms
- rough idle or shaking, often worse when cold or under load
- hesitation, jerking or loss of power when accelerating
- engine warning light; if it flashes, the misfire is severe enough to overheat the catalytic converter
- higher fuel consumption
Likely causes, most likely first
| No. | Cause |
|---|---|
| 01 | worn or overdue spark plugs |
| 02 | one or more failing ignition coils |
| 03 | vacuum leak, often a split intake hose or failed crankcase breather (PCV) valve |
| 04 | LPG system fault on converted cars (worn gas injectors, reducer or poor calibration) |
| 05 | carbon build-up on the intake valves of direct-injection petrol engines |
| 06 | low fuel pressure or poor-quality fuel |
| 07 | low compression or incorrect valve timing (stretched timing chain) |
What to check, step by step
- 01Read freeze-frame data and the per-cylinder misfire counters in live data to see whether misfires really are random or concentrated on one or two cylinders
- 02Check spark plug age and condition against the service schedule, and inspect coils and plug wells for cracks or oil
- 03Read fuel trims: strongly positive trims point to a vacuum leak or fuel delivery problem, so smoke-test the intake and check the crankcase breather
- 04On LPG-converted cars, check whether the misfire also happens on petrol; if not, inspect the gas side first
- 05Check fuel pressure against specification
- 06Do a compression or leak-down test and check cam/crank timing values if the above are fine