Misfire counts, self-test results and fuel trims

Engine health (Diagnose → Engine health) shows how many times each cylinder misfired, the engine computer's own test results with the limit it turns the light on at, and the fuel trims.

Misfire counts

Press Run health check. The Misfire counts panel lists each cylinder with an Average and a This trip count. One cylinder standing out usually points to that cylinder's coil, spark plug or injector.

With saved checkups in your garage, Misfires over time charts them, oldest checkup on the left. One number on its own means little. The same cylinder climbing is what matters. A cylinder that drops to nothing after a repair is what you hope to see - keep an eye on it over the next few drives.

Misfire counters are read on 2008 and newer (CAN) vehicles. If yours does not report them, the panel says so.

The computer's own test results

Your engine computer tests the catalytic converter, the oxygen sensors and the fuel vapour system as you drive, and keeps what it measured next to the limit it turns the light on at. Engine health shows those measurements next to their limits.

These are the vehicle's own numbers, in the vehicle's own units. A part close to its limit hasn't failed – it is telling you roughly when it will.

Fuel trims

Fuel trim is how much extra fuel the engine computer keeps having to add or take away. Near zero means the engine is behaving exactly as it expects. The panel lists what usually causes a trim that is far from zero.

Read once at idle and again while driving: a reading that is high at idle but fine under load is air getting in, and one that is high everywhere is usually fuel.