Engine misfiring: rough running and what it means

What a misfire actually means

If the mixture, the compression or the spark is missing in a cylinder, that cylinder does not burn its fuel properly. The engine computer detects it and stores a code: P0300 when it cannot pin the misfire to one cylinder, or P0301 to P0308 when it can, with the last digit giving the cylinder number.

How a cylinder is meant to fire

Every cylinder repeats the same cycle: it draws in an air-fuel mixture, compresses it, and a spark ignites it to force the piston down.

Common causes

• Worn spark plugs
• A failing ignition coil
• A fuel injector or fuel delivery issue
• A vacuum leak, meaning a cracked or loose hose letting in unmetered air
• Low compression in a cylinder, the expensive version

What to do now

1. Check whether the check engine light is flashing or steady. That sets the urgency, whatever the code.
2. If it is flashing, reduce speed and drive as little as possible.
3. Note when it runs rough: at idle, under power, and whether you have lost power.
4. Get the code read and write it down before anything else.
5. Do not clear the code until the fault is fixed.

What the light and the code tell you

• Check engine light flashing: an active misfire, reduce speed and get it checked immediately.
• Steady light with a misfire code: get it checked soon.
• P0300, random or multiple cylinders: something affecting the whole engine, like a vacuum leak or fuel supply.
• P0301 to P0308, one cylinder: that cylinder's spark plug, ignition coil or fuel injector.
• Power loss and rough idle with no obvious cause: possibly compression, and a compression test checks each cylinder.

What people often get wrong

That new spark plugs fix every misfire. Plugs and coils are among the most common and affordable causes, and worth ruling out early, but a random pattern across several cylinders often points at something affecting the whole engine, such as a vacuum leak or the fuel supply.

When the car must go to a workshop

A flashing light with a misfire code should be checked immediately. Actual diagnosis of a misfire is worth leaving to a technician with the right tools: guessing and replacing parts one at a time often costs more than a proper diagnosis.

Read next

• P0300: misfire on more than one cylinder
• P0301 to P0308: a misfire on one specific cylinder
• P0171: system too lean

This page is general information and does not replace a professional assessment. Symbols, codes and behaviour vary between makes. Always check the handbook for your own car.