Nine times out of ten a car that won't start is a flat or dying battery — especially after cold overnight temperatures or 2+ weeks parked. Test with headlights: bright = not battery, dim/nothing = battery. Read the full walkthrough below to narrow down the exact cause before spending money on the wrong part.
The 60-second at-home test that identifies 90% of no-start faults
Before you call anyone, do this: turn the ignition to position II (dashboard lights on, don't try to crank yet). Watch the dashboard. (A) Dash goes dead or dim, radio cuts out — that's battery, 95% certain. (B) Dash bright, you turn the key and hear a rapid clicking noise but no cranking — that's battery-under-load, still battery 80% of the time, occasionally starter solenoid. (C) Dash bright, engine cranks over slowly then dies — battery has enough voltage for the dash but not for the starter, still battery. (D) Dash bright, engine cranks strongly but won't fire — NOT battery. That's fuel, ignition, immobiliser or crank sensor. Fix the diagnosis before you fix the car.
Battery: the honest reality of UK car batteries
A standard lead-acid car battery lasts 4–6 years in the UK climate. AGM batteries in modern stop-start cars last 3–5 years and cost 2–3× more. Signs a battery is on borrowed time: slow crank on cold mornings, stereo forgets its stations after being parked overnight, headlights dim at idle, stop-start feature refuses to engage. Any of those and the battery is 12 months from failure. Cost to replace: £70–£120 for a standard battery fitted at an indy, £180–£280 for an AGM stop-start battery fitted and coded (modern cars need the battery registered with the ECU or the car doesn't charge it properly). Never fit an AGM battery to a car that only asked for standard, and never fit standard to a car that needs AGM — both cause chronic charging faults.
Starter motor vs solenoid vs battery — telling them apart
Same 'click and no crank' symptom, three different causes and three different price tags. Battery under load: try jump leads. If it starts immediately, it's battery. Starter solenoid failing: single loud clunk each time you turn the key, sometimes starts on the third or fourth try, gets worse over weeks. Starter motor itself: cranks fine when cold but won't crank when hot, or bench-tests fine but fails in the car. Jump-starting cures the battery diagnosis. If jump-starting doesn't cure it, you're into starter territory — £180–£350 fitted at an indy for most UK cars, more for anything with the starter buried under the intake manifold (some VAG and BMW petrol engines are notorious).
Engine cranks but won't fire — the four main causes
Strong crank, no ignition means: (1) Fuel — empty tank, failed fuel pump, or blocked fuel filter. Listen for the fuel pump priming (2-second hum) when you turn ignition to position II. Silence = pump fault. (2) Immobiliser — key not recognised, especially after a battery disconnection on modern cars. Warning symbol will be flashing. (3) Crank or cam sensor — extremely common on 10+ year old cars, especially Ford, Vauxhall and Peugeot diesels. Car will crank forever, occasionally fire briefly, then die. £60-£120 part, £80-£150 labour depending on access. (4) Timing belt or chain jump — worst case, especially on 100k+ mile cars with unknown belt history. If you smell fuel and the engine won't fire at all, stop cranking — you may have snapped the belt and continuing to crank causes valve damage.
When to attempt DIY and when to phone the garage
DIY is safe for: jump-starting, replacing a battery on a non-coded car, replacing a fuel filter on a diesel with in-line filter (not tank-integrated), checking fuses, cleaning battery terminals, checking key-fob battery. DIY is not safe for: anything requiring the car to be lifted, starter motor replacement on tight installations, coded battery replacement (modern German cars, most 2015+ Fords, hybrids), any immobiliser or key-programming fault, or any suspicion of a snapped cambelt. When in doubt, phone Bob's — a phone-call diagnosis is free, and we'll tell you honestly whether it's worth booking in or fixable in your driveway with a £30 part from the local factor.
Frequently asked questions
Why won't my car start but the lights work?
Lights work on very little current; starter needs 400+ amps. A battery can light the dash brightly but still lack the punch to crank the engine. Try jump leads — if it starts immediately, it's battery. If it doesn't, it's starter, solenoid or ignition system.
Why does my car click but not start?
Rapid clicking = battery too weak to hold contact. Single loud clunk = starter solenoid failing. Test with jump leads: if rapid-click cures instantly, battery. If single clunk persists, starter.
Can a bad alternator cause a car not to start?
Yes, indirectly. A failed alternator won't charge the battery, so the battery gradually dies until the car won't start. Symptom sequence: warning light on dash for days or weeks, then a no-start. The battery gets blamed but the alternator is the real cause. Full test: £30, replacement £250-£450 fitted.
How long can a car sit before the battery dies?
Modern cars with alarms, keyless entry and always-on modules typically flatten a battery in 2–4 weeks. Older cars (pre-2010) usually last 6–8 weeks. Fit a trickle charger if leaving a car more than a fortnight.
Is it OK to jump-start a modern car with electronics?
Yes, if you follow the sequence: black-to-black-to-earth (not battery negative on the dead car), engine running on the donor car for 5 minutes before attempting the start. Never let the leads touch. Never jump-start a hybrid or EV — different procedure entirely.
How do I get same-day help if my car won't start?
Phone Bob's workshop by 10am for a same-day diagnostic slot. If the car can't be driven to us, we'll advise on the phone whether it's safe for you to jump-start home or whether you need recovery to us — we work with local recovery firms in Perthshire.