If the battery is fine but the car won't start, the most common causes are a faulty starter motor, immobiliser fault, fuel pump failure, or crank/cam sensor failure. If the engine cranks normally but won't fire, suspect fuel or spark; if it won't crank, suspect starter, ignition switch, or earth strap.
Diagnosing a no-start usually takes a mechanic five minutes if they ask the right questions. Here's how to narrow it down before you call.
Step 1: does the engine crank?
Turn the key. Does the engine spin (cranking sound) or do you just get a click or nothing? This is the single most useful diagnostic question.
It cranks but won't fire
Engine spinning but not catching. Usual culprits: fuel pump (no fuel pressure), crank/cam sensor (engine doesn't know when to spark), spark plugs/coils (no spark), or immobiliser (key not recognised). A scan tool will pinpoint within minutes.
It won't crank at all (just clicks)
Single click usually means starter motor solenoid or low battery. Rapid clicking means battery doesn't have enough current. If battery tests good, it's usually a starter or earth strap.
Dashboard lights go dim when you turn the key
That's a battery or earth fault even if the battery looks fine on a meter — under load it's dropping out. Get a proper drop test done.
Immobiliser red light flashing fast
Key not being recognised. Try the spare key. If neither works, the immobiliser ring around the ignition has failed — common on Vauxhalls and Renaults. £80–£250 to fix.
Can a fuel pump just fail without warning? — Yes — they often die without symptoms, especially on cars over 8 years old. Listen for the 2-second hum from the back of the car when you turn the ignition to position II.
How much is a starter motor UK? — £180–£450 fitted at an independent garage depending on access.
More detail on: Why won't my car start when the battery is fine?
The short answer above covers the usual case. Below is the extra detail a UK driver normally needs to act on it — how to narrow the fault down, what it should cost, and when it is worth paying a garage rather than carrying on. This is general guidance from a working mechanic, not a substitute for someone putting your own car on a ramp.
How to describe the problem so you get a straight answer
Mechanics diagnose faster when they get facts rather than conclusions. Give the make, model, engine size, fuel type, year and mileage; say exactly when the fault happens (cold start, motorway speed, braking, turning, first five minutes); say what changed just before it started; and list any codes, warning lights or recent work. A three-line description in that shape is worth more than a paragraph of theory.
What to do before you spend money
Run a free MOT history check to see whether the same item has been advisoried before — that history often explains a fault in one line. Check whether an open safety recall covers the component, because that repair is free at a franchised dealer. Then get the fault properly diagnosed rather than replacing parts in order of price.
What Bob would check first
Start with the cheap, common and visible: fluid levels and their condition, tyre pressures and wear pattern, battery voltage and terminal condition, and every connector and vacuum pipe in the area of the fault. A surprising share of electrical gremlins are a corroded earth or a chafed loom, and a surprising share of running faults are an air leak downstream of the sensor doing the complaining. Only after that does anything get unbolted.
When it is worth going to a main dealer
Three situations justify main-dealer money: an open safety recall (free), a live manufacturer warranty or goodwill claim, and coding or programming work that needs manufacturer-level software. Everything else — servicing, brakes, suspension, clutches, cambelts, diagnostics — an experienced independent will do to the same standard for a good deal less, and using an independent does not invalidate your warranty under UK block exemption rules as long as the correct parts and schedule are used.
Safe to keep driving?
Treat anything affecting brakes, steering, tyres, seat belts or visibility as stop-now, no matter how well the car appears to be driving. Overheating and low oil pressure are also stop-now: the damage curve on both is measured in minutes, not miles. Most other faults will let you drive gently to a garage, but if the car is in limp mode, losing fluid, or the fault changes how it steers or stops, get it recovered instead. Recovery is cheaper than a tow plus a new engine.
Get an answer on your own car
If you want this looked at properly for your exact vehicle, send the details and Bob will write back — make, model, engine, mileage, when the fault happens and any codes you have. You get a written reply from a qualified UK mechanic with 32 years on the tools, not a chatbot, and your money back if he cannot help.
Ask a mechanic about your car · Free MOT history check · Free recall check · Fault code library