'My car won't start but the battery is fine' is one of the most-searched UK car problems, and the one where drivers most often waste money. If the battery has been tested (multimeter reads 12.4V+ resting, cranks the engine over strongly, jumps don't help), the fault is somewhere else — starter motor, immobiliser, fuel supply, crank sensor, security system, or a poor earth. Bob has 32 years on the tools and has seen every version of 'battery's good but it won't start'. The good news: 6 out of 10 of these are cheap to fix once diagnosed correctly. The bad news: guessing wastes hundreds. This guide walks through exactly what's failing based on what the car is doing when you turn the key.
First — confirm the battery really is good
Before assuming the battery is fine, do three quick checks. One: resting voltage after the car has sat for 4+ hours should be 12.4–12.7V on a multimeter across the battery terminals. Below 12.2V and the battery is either flat or dying — even if lights work. Two: cranking voltage should stay above 9.6V while cranking. If it drops below 9V under cranking, the battery is dying under load even if resting voltage is fine. Three: turn on headlights for 30 seconds, then try starting. If lights dim heavily as you turn the key, the battery is weak. Only once all three tests pass is the battery genuinel
What the car is doing tells you the fault
Turn key, hear nothing at all (no click, no crank, dashboard lights fine) — starter motor solenoid failure, ignition switch, immobiliser, neutral safety switch (auto), or a broken wire from the ignition to the starter. Turn key, single loud CLICK, engine doesn't crank — classic starter motor solenoid engagement failure, OR a corroded main power feed to the starter. Not the battery. £180–£380 fitted starter, or £15 for a battery terminal clean if it's corrosion. Turn key, rapid CLICK-CLICK-CLICK, engine doesn't crank — this IS a weak battery or bad connection, not a starter fault. Re-test the b
The 8 most common UK workshop causes when battery is good
1. Failed starter motor solenoid — single loud click when you turn the key, no crank. Very common on Ford Focus / Fiesta, Vauxhall Astra, Peugeot 208. Diagnostic £30–£60, replacement £180–£380 fitted. 2. Immobiliser fault or dying key fob battery — car cranks but won't fire, security light flashes. Try the spare key first (rules out key vs car fault). Common on Renault Megane / Clio, older BMW E90, Ford Mondeo. Key reprogramming £80–£180. 3. Failed crankshaft position sensor — hot-start problems that get worse over months, eventually won't start at all when hot. Common on VAG 1.6/2.0 TDI, BMW
UK diagnostic and repair pricing
Battery load test at Halfords / Kwik Fit / most independents: free–£20. Full no-start diagnostic at a UK independent (fuel pressure test, spark test, crank sensor scope, starter draw test): £60–£120. Common repair costs: battery cable / earth strap replacement £30–£80, key fob battery / reprogram £15–£180, starter motor £180–£420 fitted, crank sensor £120–£280 fitted, fuel pump £280–£620 fitted, timing chain kit £900–£1,800 fitted (much more if valves have hit pistons). If the RAC or Green Flag come to a no-start, they'll do a rough diagnosis for free as part of the callout — take their notes
When to call recovery vs try one more thing
Call recovery immediately if: the car cranks but there's a metallic knocking / grinding noise (timing chain), fuel smell in the engine bay, oil pressure warning light was on before the car cut out, or the car cut out while driving and won't restart. Try one more thing if: no click at all — check all interior lights come on with ignition, then check the fuse for the starter relay (usually 30A or 40A in the engine bay fuse box). No click AND no interior lights = battery isn't as good as you thought, retest. Rapid clicks = weak battery under load, jump-start might work. Single loud click = starte
My car battery tested at 12.6V but it still won't start — what's wrong? — Resting voltage of 12.6V is healthy, but a battery can pass a resting test and still fail under cranking load. Get a proper battery load test (free at most Halfords / Kwik Fit) — this measures voltage while cranking. If cranking voltage stays above 9.6V, the battery is genuinely fine and the fault is elsewhere: usually a starter motor, corroded main cable, or on cars that crank but won't fire, a c
Why does my car click once and not start? — A single loud click when the key is turned, with no cranking, is almost always a failed starter motor solenoid or a corroded main power cable to the starter — not a battery fault. Try tapping the starter with a rubber mallet while someone else turns the key — if it starts, the starter is on its way out. Repair cost £180–£380 fitted at a UK independent.
Could a bad key fob stop my car from starting? — Yes. If the immobiliser can't read the key, the engine will crank but not fire (or won't crank at all on some cars). The security light on the dash usually flashes. Try the spare key — if it starts on the spare, the primary key's transponder chip is failing or the fob battery is dying. Reprogram or replace: £15–£180 depending on car.
Bob's Mechanical Repairs — independent family-run garage in Birnam, Dunkeld, Perthshire. Call 01350 727 276 or email bob@bobsmechanicalrepairs.co.uk.