What does OBD code P0300 mean?
P0300 means random or multiple cylinder misfire detected. The engine's ECU has identified that one or more cylinders are misfiring irregularly. A misfire means the air-fuel mixture in a cylinder is not igniting correctly. This can cause rough running, a flashing engine management light, poor fuel economy, and in severe cases can damage the catalytic converter. The engine management light will flash if the misfire is severe enough to risk catalyst damage — stop safely if this occurs.
Likely causes
Worn spark plugs
Most common cause — old or fouled plugs cause irregular ignition
Faulty ignition coil
A coil failure causes consistent misfire on one cylinder, but can feel random if borderline
Fuel injector fault
A dirty or failing injector causes lean misfire on affected cylinder
Vacuum leak
Air leak causes lean mixture — rough idle and hesitation on acceleration
Low compression
Worn engine — compression test needed to diagnose
Repairs you may need
Typical cost range: £60–£400 — Spark plug replacement: £60–200 depending on number of cylinders. Ignition coil: £80–250. Full diagnostic: £40–80.
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