Why will my car not start?

Working out from what the car does when you turn the key whether it is the battery, the fuel, the immobiliser or something that needs a garage.

Difficulty: beginner
Time: 20 min
Read: 4 min
Safety: warning

Short answer

Listen to what happens when you turn the key. Nothing at all with a dead dashboard, or rapid clicking with the lights on, both mean the battery. An engine that turns over at normal speed but never fires is not a battery problem — check fuel, try the spare key, and check the gear selector and clutch.

A car that will not start feels like one problem but is really four or five different ones, and the noise it makes tells you which. Getting that right in thirty seconds saves an unnecessary call-out or a wasted new battery.

Safety

If you jump start, the order of connection matters and the final clamp must go to a bare metal earth point on the flat car's engine or chassis, never to the flat battery's negative terminal — a flat or charging battery gives off hydrogen, and a spark at the terminal can ignite it. Never smoke or use a naked flame near a battery, wear eye protection, and keep leads clear of belts and fans. If the car will not start on a road, in traffic or anywhere unsafe, call for assistance rather than working at the roadside.

Working out why a car will not start

Step by step

Turn the key and listen carefully.Silence, a single click, rapid clicking, slow laboured cranking and normal cranking are five different symptoms with five different causes. This is the whole diagnosis.
If the dashboard is completely dead, look at the battery terminals.A totally dead car is usually a flat battery or a loose or corroded connection. Terminals should be tight and clean; a white or blue crust means corrosion.
If you get rapid clicking, treat it as a flat battery.There is enough charge to work the relay but not the starter motor. This is the classic case for a jump start or a battery booster pack.
If it cranks slowly, suspect a weak battery or the cold.A battery that is on its way out often manages on a mild day and fails on the first frost. Have it tested rather than waiting for it to strand you.
If it cranks normally but never fires, stop thinking about the battery.The starter is doing its job. The engine is not getting fuel, or spark, or is being prevented from running by the immobiliser.
Check the immobiliser and the key.Try the spare key. A key fob with a dying battery, or a transponder that has not been recognised, will crank the engine but not let it run. Some cars show a key symbol on the dash.
Check the obvious interlocks.An automatic will not start unless it is in Park or Neutral — try Neutral. A manual usually needs the clutch fully down. Both catch people out surprisingly often.
Check there is actually fuel.Gauges fail, and a car parked on a slope can read differently. If it has recently been filled, consider whether the wrong fuel went in — in which case do not crank it any further.
Stop cranking after a few attempts.Repeated long cranks flatten the battery, overheat the starter and, on a petrol engine, flood it. Three or four tries is enough to know it is not going to start.
Call for assistance if the checks do not resolve it.Fuel systems, ignition and starting circuits are beyond an owner check. Recovery is cheaper than the damage caused by continued cranking or by an improvised repair.

Tips

A battery that needs jump starting more than once is a battery that needs testing, not another jump.
Switch off the heater, lights, heated screen and radio before trying again — everything electrical takes charge away from the starter.
On a diesel in cold weather, wait for the glow plug light to go out before cranking.

Common mistakes

Cranking repeatedly in the hope it catches — It flattens the battery, can overheat the starter motor, and on petrol engines floods the cylinders. If it has not started in three or four attempts, something is wrong that cranking will not fix.
Buying a new battery because the car will not start — A car that cranks at normal speed has a battery that is doing its job. Replacing it costs money and changes nothing.

If it doesn't work

Everything dead, no interior lights

Cause: Flat battery or a disconnected terminal — Fix: Check the terminals are tight and clean, then jump start or call for assistance.

Rapid clicking, lights dim as you turn the key

Cause: Battery too flat to turn the starter — Fix: Jump start with correct lead order, then drive for a good half hour or put it on a charger.

Cranks well, will not fire, key symbol showing

Cause: Immobiliser not recognising the key — Fix: Try the spare key. If that works, the first key's transponder battery is likely dead.

Started fine yesterday, dead this morning

Cause: Something drawing current overnight, or a failing battery — Fix: Check for an interior light or boot light left on, then have the battery and the parasitic drain tested.

Questions people ask

How do I tell a flat battery from a starter motor fault?

A flat battery gives dim lights and rapid clicking; the whole car droops when you turn the key. A failed starter usually gives one solid click with the lights staying bright, because the electrics are healthy and the motor is not.

Can I push start a modern car?

Only a manual, and only if the immobiliser and electronics are working. It will not work on an automatic, and it will not work on a car whose battery is too flat to power the engine management.

How long should I drive after a jump start?

At least twenty to thirty minutes of steady driving, ideally more, and preferably not all stop-start. A short trip afterwards puts back less than the start took out.