Car Won't Start: Battery, Starter Motor Or Alternator?
Car Parts Centre · 1 August 2026
Five-minute diagnosis using sounds, dash lights and a multimeter before you spend a penny on parts.
Most no-start calls turn out to be one of three things. You can separate them at the roadside with your ears, the dashboard, and a cheap multimeter — long before ordering parts.
Listen first
Turn the key and note exactly what happens. The sound narrows the fault faster than anything else.
- Rapid clicking, lights dim heavily: flat or failing battery, or a poor earth/terminal connection.
- One solid clunk, no crank, lights stay bright: starter motor or solenoid.
- Cranks strongly but won't fire: fuel, ignition or crank sensor — not the battery.
- Nothing at all, no dash lights: main earth, battery terminal or immobiliser.
Battery voltage figures
A multimeter across the terminals tells you most of what you need. Test with everything off after the car has stood for an hour.
- 12.6V+ — fully charged.
- 12.4V — around 75%; will struggle in cold weather.
- 12.0V or less — discharged; charge and retest before condemning.
- Below 9.6V while cranking — battery cannot deliver current, replace it.
- 13.8–14.7V with the engine running — alternator charging correctly.
Signs the alternator is at fault
A battery warning light that appears while driving, headlights that pulse with engine speed, or a battery that goes flat again within a day of charging all point at charging-system failure rather than the battery itself. Check the auxiliary belt and tensioner at the same time — a glazed or slipping belt produces identical symptoms.
Before you buy a battery
Match the case size, terminal layout and cold cranking amps to the original, and check whether your car needs AGM or EFB. Stop/start vehicles need the correct type or the battery will fail within months, and many require the new battery to be registered to the vehicle.
Frequently asked questions
How long should a car battery last?
Typically four to six years in UK conditions; short journeys and stop/start use shorten that.
Can I fit an EFB battery where AGM was fitted?
No. You can replace EFB with AGM in most cases, but not the other way round.
Does jump starting damage anything?
Not if done in the correct order and with the donor engine running at idle. Never disconnect a battery to 'test' the alternator on a modern car.