Car won't start? Find the likely cause
What the car does when you turn the key tells you more than anything else. Completely dead or rapid clicking almost always points to the battery. Turning over normally without firing points to fuel, ignition or a sensor. Starting and then dying straight away is often the immobiliser not recognising the key.
Work through the checks below before paying for a call-out — many cars that won't start can be diagnosed, and often fixed, where they're parked.
Free interactive diagnosis
Is it petrol or diesel?
A hybrid counts as petrol. The likely causes are completely different between the two.
Is it safe to keep driving?
For a typical car with each symptom. These are general guidance, not a diagnosis — if you're unsure, don't drive it.
- Car completely dead — no lights, no soundTry simple checks
- Car clicking but won't startTry simple checks
- Car struggling to start (turning over slowly)Try simple checks
- Car turns over but won't startTry simple checks
- Car starts, then diesTry simple checks
Car completely dead — no lights, no sound
Try the simple checks first
Most cars that won't start can be diagnosed, and often fixed, where they're parked. Work through the checks below before calling anyone out. If it does start, get it looked at soon — a car that failed to start once can fail again somewhere less convenient.
Likely causes
- Flat or failing 12V batteryMost likely
The dash and interior lights are dim or die when you try to start. If a jump start fires it straight up, you've found it.
If it's this: usually a simple fix.
- Loose or corroded battery terminalsLikely
Pop the bonnet and try to twist each battery clamp by hand. White or green crust on the posts means poor contact — a wire brush fixes it for free.
If it's this: usually a simple fix.
- Key battery or immobiliser not recognising the keyPossible
A key or padlock symbol flashes on the dash. Try the spare key before paying anyone.
If it's this: usually a simple fix.
- Hybrid or electric-car 12V or high-voltage system faultPossible
The READY light won't come on, or a hybrid system warning shows. The 12V battery is the most common culprit; beyond that it's dealer diagnosis.
If it's this: usually needs a garage.
What to do next
Try the spare key, then a jump start, before spending any money. If it starts on a jump, the car is fine to drive: keep the engine running, drive for at least 30 minutes so the battery recharges, and don't switch off anywhere you'd be stuck if it didn't restart. Then get the battery and charging system tested — most motor factors do it free.
Car clicking but won't start
Try the simple checks first
Most cars that won't start can be diagnosed, and often fixed, where they're parked. Work through the checks below before calling anyone out. If it does start, get it looked at soon — a car that failed to start once can fail again somewhere less convenient.
Likely causes
- Flat or failing 12V batteryMost likely
The dash and interior lights are dim or die when you try to start. If a jump start fires it straight up, you've found it.
If it's this: usually a simple fix.
- Starter motor or solenoidLikely
One firm click, the dash stays bright, and a jump start makes no difference.
If it's this: usually needs a garage.
- Loose or corroded battery terminalsLikely
Pop the bonnet and try to twist each battery clamp by hand. White or green crust on the posts means poor contact — a wire brush fixes it for free.
If it's this: usually a simple fix.
What to do next
Rapid clicking is almost always the battery — try a jump start. If it starts on a jump, it's fine to drive, but get the battery tested before relying on it. A single firm click with bright dash lights points at the starter motor, which a jump start won't fix — that needs a garage or recovery.
Car struggling to start (turning over slowly)
Try the simple checks first
Most cars that won't start can be diagnosed, and often fixed, where they're parked. Work through the checks below before calling anyone out. If it does start, get it looked at soon — a car that failed to start once can fail again somewhere less convenient.
Likely causes
- Flat or failing 12V batteryMost likely
The dash and interior lights are dim or die when you try to start. If a jump start fires it straight up, you've found it.
If it's this: usually a simple fix.
- Glow plugs or glow plug relayPossible
Hard to start when cold, fine once warm, and it gets worse week by week. A garage can test each plug in about ten minutes.
If it's this: usually needs a garage.
- Starter motor or solenoidPossible
One firm click, the dash stays bright, and a jump start makes no difference.
If it's this: usually needs a garage.
- Alternator not chargingPossible
The red battery light is on while driving. With the engine running, a meter across the battery should read about 13.8–14.7V; much lower means it isn't charging.
If it's this: usually needs a garage.
What to do next
Try a jump start. If it starts, it's fine to drive — keep it running for at least 30 minutes. If it happens again the next morning, have the battery and alternator tested together — a weak alternator kills new batteries.
Car turns over but won't start
Try the simple checks first
Most cars that won't start can be diagnosed, and often fixed, where they're parked. Work through the checks below before calling anyone out. If it does start, get it looked at soon — a car that failed to start once can fail again somewhere less convenient.
Likely causes
- Fuel pump, fuel filter or air in the fuelMost likely
On a petrol car, listen for a two-second hum from the back seat when you switch the ignition on — silence points at the pump. Rule out an empty tank and a faulty fuel gauge first.
If it's this: usually needs a garage.
- Glow plugs or glow plug relayLikely
Hard to start when cold, fine once warm, and it gets worse week by week. A garage can test each plug in about ten minutes.
If it's this: usually needs a garage.
- Ignition coil or spark plugsPossible
A misfire code (P0300–P0304), shaking at idle, and a loss of power. Swapping a coil to another cylinder and seeing the fault move confirms it.
If it's this: usually needs a garage.
- Crankshaft position sensorPossible
It turns over normally but never catches, and the rev counter doesn't move while cranking. It usually stores a fault code.
If it's this: usually needs a garage.
What to do next
Don't keep cranking for long — it flattens the battery and, if the timing belt has gone, can do more damage. Check there's fuel and look for a flashing key symbol, then have the fault codes read.
Car starts, then dies
Try the simple checks first
Most cars that won't start can be diagnosed, and often fixed, where they're parked. Work through the checks below before calling anyone out. If it does start, get it looked at soon — a car that failed to start once can fail again somewhere less convenient.
Likely causes
- Key battery or immobiliser not recognising the keyMost likely
A key or padlock symbol flashes on the dash. Try the spare key before paying anyone.
If it's this: usually a simple fix.
- Dirty throttle body or idle controlLikely
It fires, then dies unless you hold the revs up. Cleaning the throttle body is often all it needs.
If it's this: usually a simple fix.
- Fuel pump, fuel filter or air in the fuelLikely
On a petrol car, listen for a two-second hum from the back seat when you switch the ignition on — silence points at the pump. Rule out an empty tank and a faulty fuel gauge first.
If it's this: usually needs a garage.
- Air flow sensor or a split boost hoseLikely
A flat spot or limp mode, codes P0100–P0104 or a boost fault. A split intake hose often hisses under load.
If it's this: usually needs a garage.
What to do next
Watch for a flashing key or padlock light as it dies — that's the immobiliser, and the spare key will tell you. Otherwise have the fault codes read.
A shortlist, not a diagnosis
The ranking is the one a mechanic would give over the counter for a typical car — nobody can road-test a car over the internet, and the real cause may not be listed. Add the registration in the checker above to have it checked against that car's MOT advisories, mileage and engine. Have the car checked by a qualified mechanic before relying on it, and if you're unsure whether it's safe, don't drive it.
Common questions
Why won't my car start when the battery seems fine?
If the dash lights stay bright and the engine turns over at normal speed, the battery is doing its job and the problem is fuel, spark or a sensor — most often the fuel pump, the crankshaft sensor, or glow plugs on a diesel in cold weather. If it doesn't turn over at all, the lights stay bright and you hear one firm click, suspect the starter motor.
Why won't my car start in the cold?
Cold weather cuts a battery's output and thickens the oil, so a tired battery often gives up on the first frosty morning. On a diesel, worn glow plugs are the other usual cause — hard to start when cold, fine once warm.
Should I keep trying to start it?
No more than a few short attempts. Long cranking flattens the battery and, if the timing belt has failed, can do more damage. If it turns over but won't fire after a few tries, stop and have it looked at.
Is it safe to drive after a jump start?
Usually, yes. Keep the engine running and drive for at least 30 minutes so the battery recharges, and don't switch off anywhere you'd be stuck if it didn't restart. Then have the battery and charging system tested — a weak alternator will flatten a new battery too.
Related symptoms
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Running poorly
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Noises
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All symptoms: Engine management light · Other warning lights · Exhaust smoke · Running poorly · Leaks · Brakes · Steering & vibration · Noises · Symptom checker