Starts then dies within seconds: immobiliser, fuel or idle control

An engine that fires and immediately stops had everything it needed for combustion for a moment, and then lost one of them. That is a much narrower problem than an engine that will not fire at all.

Time it. Two seconds and one minute point in completely different directions.

Related: car won’t start diagnosis and engine cranks but won’t fire.

Runs for one to two seconds: the immobiliser

The classic signature.

Many immobiliser implementations allow the engine to fire on residual rail pressure, then cut fuel once the module confirms the key is not authorised.

The tell: a security or key symbol on the dashboard, sometimes flashing.

Causes: a failed transponder in the key, a faulty antenna ring around the ignition, a key that has lost its programming, or an aftermarket alarm interfering.

Car won’t start: immobiliser troubleshooting and immobiliser key learning per brand.

Try the spare key. If it starts and runs, the first key is the fault — and that is a much cheaper answer.

Runs for a few seconds then dies: fuel supply

The engine runs on the pressure the pump built during priming, then stops when that pressure is used and not replaced.

The listening test: ignition on without cranking. A healthy system runs the pump for one to three seconds. Silence means no pump.

Causes:

• The fuel pump relay or its circuit: P0230 to P0232 fuel pump primary circuit.
• The fuel pump control module on newer vehicles: P0627 to P0629 fuel pump control circuit.
• A blocked filter or a failing pump.
• The inertia switch, tripped by a pothole or a knock. Free to reset, and worth knowing your vehicle has one.

Log fuel rail pressure during the attempt. Pressure that builds and then falls away is the fault, recorded: P0087 fuel rail pressure too low.

Runs then dies as it drops to idle: idle control

The engine starts on the enrichment strategy, then stalls the moment it tries to hold idle.

Causes:

• A dirty throttle body, particularly after cleaning without an adaptation relearn: P2176 throttle idle position not learned.
• Idle control faults: P0506 idle speed lower than expected and P0505 idle air control diagnostics.
• A large vacuum leak: P2279 intake air system leak.

The test: hold a little throttle. If it keeps running with your foot on the accelerator and dies when you lift, it is idle control.

Runs and dies unpredictably: sensor dropout

A crankshaft or camshaft sensor that fails intermittently, often when hot.

The signature: it starts fine cold, dies after a while, refuses to restart, and starts again after half an hour. Owners describe it as needing to rest.

P0320 to P0322 engine speed input, P0336 crankshaft sensor range/performance and engine stalls while driving causes.

Anti-theft aftermarket devices

Alarms and remote starters spliced into ignition, fuel or starter circuits are a recurring cause of exactly this symptom — particularly ones fitted years ago whose wiring has aged.

If the car has an aftermarket alarm or tracker, it belongs high on the list: alarm and tracker installation and aftermarket accessories electrical faults.

What to record

The diagnosis lives in the timing and the accompanying signs:

Observation	Points at
Security lamp on the dashboard	Immobiliser
Runs with throttle held	Idle control or a vacuum leak
Dies after several seconds, no pump noise	Fuel supply
Hot only, recovers after cooling	Sensor dropout
Started after work on the car	Whatever was disturbed
Started after a pothole	Inertia switch

Where to start

1. Time it — one second, five seconds, or longer?
2. Look for a security lamp.
3. Try the spare key.
4. Listen for the fuel pump prime.
5. Try holding a little throttle.
6. Read all codes, including pending.
7. Log rail pressure and RPM during an attempt.

Try it on the free tier of the app — pair an ELM327 adapter and log RPM and fuel rail pressure during a failed start. Whether pressure built at all separates half the causes in this article from the other half.