P0300Random/Multiple Cylinder Misfire Detected
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Matters for the ITP
- Can trigger limp mode
- Needs a drive cycle to confirm a fix
- How the car detects it
- The engine module detects misfire from crankshaft speed: a cylinder that does not fire gives no acceleration impulse. At a catalyst-damaging rate (CARB: at least 5 % per 200 revolutions) the warning light flashes; at an emissions-relevant rate (at least 1 % per 1,000 revolutions – GM about 2–5 %, Ford 0.9–1.5 %) it comes on after two trips. Ford and GM may shut off a misfiring cylinder's injector to protect the catalyst. P0300 sets when the misfire cannot be assigned to one cylinder: several cylinders each account for more than 10 %, the cylinder changes randomly, or – at Ford – the cylinder cannot be identified at high speed. It often appears together with cylinder codes. Causes are usually systemic: false air, worn plugs on several cylinders, fuel pressure or fuel quality, valve timing – and false detection from a damaged tone wheel, rough roads or vibration. Bulletins name coils, cracked plug insulators and wrong plug gaps.
- What you may notice
- Check engine light on, flashing at a catalyst-damaging rate
- Rough idle, jerking, vibration
- Loss of power, hesitation
- Smell of unburnt fuel, higher consumption
- Likely causes
- LikelyFalse air (vacuum leak, crankcase ventilation, intake gasket)
- LikelyWorn or wrong spark plugs on several cylinders, faulty ignition coils
- PossibleFuel pressure too low or high, contaminated fuel
- RareValve timing (stretched chain), valves, compression on several cylinders
- RareFalse detection: tone wheel or pulley, rough road, vibration, software
- Inspection (ITP)
- Matters for the ITP. While the check-engine light is on, the car fails the emissions check.
- Repair
- Medium · Often possible to do yourself.Typical cost: €30 – €1,500
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.