Engine keeps running after you switch off: what causes run-on

Run-on — sometimes called dieseling — is an engine continuing to fire after the ignition is switched off. It happens when the mixture ignites from heat and compression rather than from a spark.

On a carburetted engine it was common. On a modern fuel-injected engine it should be impossible, because switching off cuts the injectors — and that makes it worth investigating rather than tolerating.

Related: engine knocking noise diagnosis.

Why a modern engine should not do it

Switching off the ignition stops the injectors. No fuel is delivered, so there is nothing to ignite regardless of how hot the combustion chamber is.

If a modern engine runs on, fuel is still entering the cylinders. That is the finding.

Causes:

Leaking injectors

An injector that does not seal continues to drip after the signal stops. Combined with hot combustion chambers, that can sustain firing briefly.

Usually accompanied by other symptoms: hard hot starting, a fuel smell, and rough running immediately after start.

P0201 to P0206 injector circuit diagnostics and hard starting when hot.

Carbon deposits creating hot spots

Heavy carbon build-up in the combustion chamber retains heat and creates glowing spots that can ignite mixture without a spark.

That also causes pre-ignition and detonation while running, which is more damaging than the run-on itself.

GDI carbon buildup and knock sensor diagnostics P0325.

An engine running very hot

Overheating raises combustion chamber temperatures and makes pre-ignition more likely.

Engine overheating diagnosis and P0217 engine over-temperature.

Fuel of the wrong grade

Low-octane fuel in an engine that needs higher octane detonates more readily. On a high-compression engine that is a real cause.

Bad fuel quality OBD symptoms and fuel quality and wrong fuel.

The ignition switch or relay

If the main relay does not open, the fuel pump and injectors stay powered after switch-off.

That is an electrical fault with a wider consequence: modules that never sleep also flatten the battery: P0685 to P0687 ECM power relay circuit and parasitic battery drain diagnostics.

Things that are not run-on

Worth separating, because they are normal and cause unnecessary worry.

The cooling fan running after switch-off. Entirely normal after a hard drive or in hot weather. It runs for a few minutes and stops.

The turbo timer effect on some vehicles, where an electric coolant pump continues to circulate.

A diesel particulate filter regeneration continuing, on vehicles that manage the end of a regeneration cycle after shutdown.

Hybrid systems where the engine starts and stops on its own logic. That is the system working: plug-in hybrid diagnostics.

Noises from the EVAP system or the fuel tank as things cool and contract.

None of those involve the engine actually firing. Run-on means combustion, and it is unmistakable — the engine shudders and shakes rather than humming quietly.

Stopping it in the moment

If the engine runs on:

On a manual: select a high gear, hold the brake firmly, and release the clutch. The engine stalls immediately.

On an automatic: it is harder. Applying the brake firmly in Drive may stall it, or it may simply run itself out.

Do not repeatedly try the ignition. Find the cause instead.

Why it matters

Run-on is uncontrolled combustion. The same conditions that produce it — hot spots, wrong octane, heavy carbon — produce detonation while the engine is running, and detonation damages pistons and bearings.

Treat it as a symptom of something that is also happening at full throttle, where the consequences are worse.

In what order

1. Confirm it is combustion, not a fan or a pump.
2. Check for stored codes, including misfire and knock.
3. Check ignition timing behaviour under load — retreating timing means knock correction: knock sensor diagnostics.
4. Check the fuel grade you have been using.
5. Check coolant temperature and the cooling system.
6. Investigate the injectors if hot starting is also poor.

Try it on the free tier of the app — pair an ELM327 adapter and log ignition advance and coolant temperature under load. Run-on is usually the visible end of a detonation problem that starts long before you switch off.