P0300 Random/Multiple Cylinder Misfire Detected
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.
Likely causes
Symptoms
- Check engine light on, flashing at a catalyst-damaging rate
- Rough idle, jerking, vibration
- Loss of power, hesitation
- Smell of unburnt fuel, higher consumption
Repair estimate
Typical range across vehicles; the real cost depends on what the testing finds.
What to check first
- A flashing light means stop driving hard - unburned fuel can overheat the catalytic converter
- Check misfire counters per cylinder
- Inspect plugs and coils
- Check fuel trims and pressure
Parts involved
Related codes
References: law.cornell.edu, gsi.ext.gm.com, newtonnet.co.uk, saabwisonline.com.