P0300
Random/Multiple Cylinder Misfire Detected
The ECU has detected misfires that are not concentrated on a single cylinder. The misfire monitor measures crankshaft acceleration after each combustion event; missing acceleration on multiple cylinders, or randomly across cylinders, triggers this code.
- Check engine light: Yes
- Emissions related: Yes
- Repair difficulty: medium
What we see in the workshop
ON FORD ECOBOOST
Random misfires on a direct-injected EcoBoost are very often carbon build-up on the intake valves, because there is no fuel wash to keep them clean. We scope the intake before replacing coils and plugs in sequence.
ON ALFA ROMEO
On the 1.4 MultiAir, random misfires frequently trace back to the MultiAir actuator unit rather than the ignition side.
Symptoms
- 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
Common causes
Ordered by how often each turns out to be the cause.
- Worn spark plugsCommon
- Vacuum leak affecting all cylindersCommon
- Fuel pressure lowPossible
- Timing chain stretched (high mileage)Less likely
Parts involved
- Ignition Coil
- Spark Plug
- Fuel Injector
- Intake Manifold
- Timing Chain
P0300 is a generic OBD-II code defined by SAE J2012. Exact wording and diagnostic procedure vary by manufacturer and model. Code definitions are drawn from the public-domain OBDex dataset; the workshop notes are our own.