Car Won't Start: A Structured Diagnosis

Safety first

• If you have stopped in a live lane, on a motorway or in any exposed position, leave the vehicle if it is safe to do so, get behind a barrier and call for assistance
• Do not attempt jump starts or checks under the bonnet at the roadside in traffic
• Keep hazard lights on and, where safe, use a warning triangle away from motorways
• If you smell fuel or see smoke, do not attempt to start the car again

1. Nothing happens, or a single click

The starter is not turning the engine. Attention usually goes to the battery, its connections, the immobiliser and security system, the starter circuit, or an interlock such as clutch or brake pedal switches and the park/neutral position on automatics.

• Are dashboard lights bright, dim or absent when you turn the key?
• Does the car have any electrical function at all - lights, central locking, dashboard?
• On an automatic, try selecting neutral; on a manual, press the clutch fully
• Check for a security or immobiliser message, and try a spare key if you have one

2. The engine turns over slowly

A laboured crank usually points at the battery, its state of charge or connections, though heavy engine drag and starter faults can produce the same complaint. Repeated attempts will usually make it worse rather than better.

• Common in cold weather, after short journeys, or on a battery several years old
• Corroded or loose terminals can mimic a failed battery
• A jump start that works confirms very little on its own - the battery may still need testing and charging
• Ask for a proper battery and charging system test rather than replacing on assumption

3. It cranks normally but will not fire

The starter is doing its job, so attention moves to fuel, spark or ignition on petrol engines, and to fuel supply and glow plugs on diesels - plus sensors that tell the engine management system the engine is turning.

• Is there fuel in the tank, and was the correct fuel put in last time?
• Any warning lamps illuminated with the ignition on, and do they behave normally?
• Has the car been standing for a long period?
• Stop after a few attempts - continued cranking risks flooding the engine and flattening the battery

If you suspect misfuelling, do not start the engine or turn on the ignition; call for assistance.

4. It starts then immediately dies

A brief run followed by a stall suggests the engine can fire but something is cutting fuel, air or the ability to keep running - including immobiliser interaction, fuel supply and pressure, idle control, air leaks or a sensor fault.

• Note whether it dies instantly or after a few seconds
• Note whether it will run with light throttle held
• Note any warning lamps that appear as it stalls
• Avoid repeated cycles: the information you have already is more useful than another attempt

Who to call, and what to tell them

For a no-start on a driveway or in a car park, a mobile mechanic or breakdown service can often diagnose in situ. For a no-start in an unsafe location, or where the car will need workshop equipment, recovery to a garage is usually the better route.

• Say which of the four behaviours applies, in those terms
• Say when it last ran normally and what has changed since
• Mention warning lamps, unusual noises, smells or recent work
• Say where the car is and whether it is in a safe location