Why a Car's Engine Stops When You Turn the Key Off

An engine that keeps running briefly after the key is turned off, or that dies before the ignition is fully in the off position, is an unusual fault and one that is worth understanding before anything is replaced. In most cases it is not a fault in the engine at all but a wiring or switch problem, and it is frequently caused by something that was added to the vehicle.

This guide covers the causes.

Why It Normally Stops Immediately

The ignition switch controls several circuits. In the off position it opens the supply to the ignition and the fuel system, which is what stops the engine. It also opens the supply to the starter, the accessories, and various other circuits. The engine stopping the moment the switch reaches off depends on that switch actually interrupting the supply to the fuel system and the ignition.

Where the engine keeps running, one of those supplies is being held on by something other than the switch.

The Causes

An Accessory That Has Been Wired Directly

The most common cause by a wide margin, and almost always something added rather than something original. An alarm, a tracker, a dash camera, a stereo, an amplifier, a phone charger, or a lighting accessory that has been connected to a permanent supply rather than a switched one keeps a circuit live with the ignition off.

Where the accessory draws current, the battery drains, and the vehicle may also struggle to start. Where it is wired in a way that holds an ignition circuit on, the engine continues to run. The question is worth asking of anyone who has worked on the vehicle: what was connected, and to which supply.

A Sticky Ignition Switch

Where the switch has worn internally, it may not fully interrupt the supply to the ignition or the fuel system even when it appears to be in the off position. This is more common on older vehicles where the switch has a large current to interrupt. The symptom is that the engine stops only when the key is pulled further or when the switch is jiggled, and it typically develops gradually.

A Short Circuit in the Ignition Circuit

Where a wire that should be switched has become shorted to a permanent supply through damaged insulation or through a chafed harness, that circuit stays live. This is a genuine electrical fault and it is worth finding, because a short that starts as an intermittent can become a fire risk. A burning smell from the steering column when the key is turned off is the version to take seriously.

A Modified or Repaired Ignition Switch

Where a previous owner has bypassed the switch, fitted a different switch, or fitted an auxiliary start button that holds a relay on, the original switching may no longer do what it is supposed to. That is common on modified vehicles and it is worth understanding what has been done before diagnosing anything.

An Immobiliser That Has Failed

On many vehicles the immobiliser cuts the fuel supply when the key is turned off. An immobiliser module that has failed can hold that supply on, or it can fail in a way that allows the engine to continue running. Where the vehicle has an immobiliser, this is worth checking, and on some vehicles it requires a diagnostic tool rather than a test.

A Fuel Pump Relay That Has Failed Closed

Where the relay that supplies the fuel pump is stuck, the pump continues to run after the key is off. On many vehicles the fuel pump noise is audible, and the battery will also drain because the pump draws current. This is a known failure mode on certain models.

Poor Grounding Causing a Ringing Circuit

Where the ignition circuit's ground is poor, the ignition can continue to receive a signal after the switch is off, because the reference is not switching cleanly. It is less common than the others but it does happen, particularly on a vehicle with a discharged or damaged battery.

The Battery Consequence

Where the engine continues to run, the fuel pump and the ignition are still drawing current, so the battery is being drained with the vehicle parked. Where this happens repeatedly, the vehicle will be found flat, and the driver will assume the battery is at fault. The battery is usually entirely serviceable and the real fault is the circuit that is being held on.

That makes this an important fault to investigate rather than to work around, because replacing the battery does not fix it. The measurement that identifies a drain is in how to stop a car battery from dying.

The Safety Concern

A vehicle that continues to run when the key is off is running unattended, which has two aspects. The first is that it may be running indoors or in a garage, and carbon monoxide from a running engine in an enclosed space is genuinely dangerous. The second is that it may move: on a vehicle with an automatic transmission, releasing the selector out of park with the engine running will move the vehicle, and there are vehicles where a gear selector or a shifter mechanism has failed in exactly that way.

Where the engine runs on after the key is out, it should not be left unattended until the fault is fixed.

Diagnosing It

Establish whether the engine actually stops on its own after a delay, or whether it continues indefinitely. An engine that continues indefinitely means a supply is being held on. An engine that runs for a moment and then stops suggests a residual fuel pressure in the system or a control unit that is completing its cycle, which on some vehicles is normal.

Then check what is running. With the engine still running after the key is out, open the bonnet and listen for the fuel pump, and look for the relay that supplies it. A fuel pump that is audibly running identifies the circuit immediately.

Where the engine stops, check the standing current after a period of standing. An abnormal draw identifies the circuit, and it can then be traced by pulling fuses one at a time until the draw stops, which is a direct and reliable method. The principles are in why a car's battery light comes on for the electrical system and why a car's engine light comes on with no symptom for the control unit side.

Repairing It

Where the cause is an accessory that has been wired to a permanent supply, reconnect it to a switched supply. That is a small job and it resolves the fault and the battery drain together.

Where the cause is a sticky switch, the switch is replaced. Where it is a short, the damaged wiring is found and repaired, and the harness damaged area is protected properly so it does not recur. Where it is the fuel pump relay, the relay is replaced.

Prevention

Nothing about normal maintenance prevents this. What does help is knowing what has been added to the vehicle and by whom, keeping a record of any electrical work, and using an accessory that draws its supply from a circuit that the manufacturer intended for it rather than from a permanent live. On a vehicle that has been modified, an isolation switch fitted to the accessory circuit resolves the problem permanently and cheaply.

If this guide was helpful, you may also want to read why a car dies when you turn the key for the opposite symptom, why a car's radio cuts out for the electrical noise that often accompanies a related fault, and dashboard warning lights explained to see what else is indicating an electrical problem.