Misfires: finding the cylinder

A misfire is a cylinder that doesn’t burn its fuel properly. The engine computer notices it, and often says which cylinder it was. From there, a few simple checks narrow down the cause.

Updated 16 October 2026

What a misfire is

When the fuel in a cylinder doesn’t burn properly, that cylinder doesn’t push the crankshaft round as it should. The engine computer notices the tiny dip in the crankshaft’s speed and counts how often it happens while the engine runs, except in conditions where it can’t judge reliably, such as on very rough roads.

You may feel it as shaking, an uneven idle or hesitation, but mild misfires can go unnoticed.

Why a flashing light means stop

Unburnt fuel from a misfiring cylinder goes into the exhaust and burns inside the catalytic converter, which can overheat and be damaged. When misfires are bad enough to do that, the check engine light flashes.

Flashing

Severe misfires. Stop when it’s safe to do so.

Steady

Misfires confirmed, but milder. Avoid hard driving and have it checked soon.

• P0300 to P0312: engine misfire codes

What the codes say

P0301 to P0312 name the cylinder: the last two digits are its number. P0300 means misfires in more than one cylinder, or the computer couldn’t tell which. Misfires in several cylinders point to something they share, such as a vacuum leak or the fuel supply.

Where cylinder 1 is depends on the engine. The service information for your car shows the numbering.

Look at the other codes too. Lean codes (P0171, P0174) point to air leaks or fuel delivery; camshaft timing codes (P0011, P0016) to the valve timing.

Misfire counts per cylinder

Some cars, mostly newer ones, also report how many misfires each cylinder had in recent drives, as on-board test results (service 06). These counts can show a cylinder that misfires before it’s bad enough to set a code.

Not every car reports them, and older cars often don’t. Where the car does, Read My Car shows them under Vehicle, Self-test results: the misfires per cylinder this drive, and the average of recent drives.

• Open Read My Car

The coil and plug swap

On engines with an ignition coil per cylinder, swapping parts between cylinders shows whether the part or the cylinder is at fault.

1. Switch the engine off and let it cool down. Never pull a coil or its plug with the engine running.
2. Swap the coil of the misfiring cylinder with the one of a neighbouring cylinder.
3. Clear the codes and drive normally until the misfire shows again.
4. If the code moves to the other cylinder, the coil is at fault. If it stays, do the same with the spark plugs.
5. If the misfire stays in the same cylinder after both swaps, have the injector and the compression of that cylinder tested.

Diesels have no spark plugs or coils: a misfire there points to an injector, the compression or the fuel supply.

Questions