Car Won’t Start: How to Work Out Why

What the car does when you turn the key narrows this down enormously, and it takes ten seconds to establish. Find your symptom here first:

Two free checks before anything else

The headlight test. Switch the headlights on and watch them while someone turns the key. If they are bright and collapse to almost nothing as you crank, the battery or its connections cannot deliver current. If they stay bright and all you get is a click, the battery is fine and the starter circuit is not.

The terminals. Look at the battery posts. Loose clamps, or white-green furry corrosion, will produce a perfect imitation of a dead battery and a dead starter both. Wiggle each clamp by hand — if it moves at all, that may be your entire fault, and it costs nothing.

It cranks normally but will not fire

This is the useful branch, because it rules the battery out completely. An engine needs fuel, spark and correct timing, and it will crank happily without any of them.

The immobiliser. Look for a key or padlock symbol lit or flashing on the dashboard. Try the spare key. This one costs nothing and solves it more often than people expect — a failed transponder or a flat key battery on a keyless system will crank the engine and refuse to let it run. On keyless cars, try holding the fob physically against the start button; most have a designated backup position.

Fuel. Check the gauge, and be honest about whether the sender could be lying. Then ask whether anything has changed — if the car was filled recently and would not start afterwards, misfuelling is a real possibility. If you suspect the wrong fuel, do not keep cranking; every turn pushes it further through the system and multiplies the bill.

Spark and timing. A failed crankshaft position sensor is a classic no-start with perfectly normal cranking, and it has a distinctive pattern: it often works from cold and refuses once the engine is hot, because the fault is heat related. If your car starts fine in the morning and strands you after a run, mention that.

The quiet, fast crank. If the engine spins over faster and more smoothly than usual, almost like it is freewheeling, stop. That can mean there is no compression because the timing belt has snapped, and continuing to crank a broken-belt engine can turn a repair into a replacement.

Diesels in cold weather

Two specifically diesel problems, and both are seasonal.

Glow plugs. A diesel relies on compression heat to ignite, and below about 5°C it needs help. Failing glow plugs give you hard starting when cold, a lot of white smoke, and a rough first minute — while the car starts perfectly in summer. A glow plug light that flashes rather than going out is telling you directly.

Running the tank dry. A diesel that runs out of fuel draws air into the system, and on many cars it will not simply restart once you have put fuel in — it needs bleeding. If you ran it to empty, that is your answer.

What not to do

• Do not crank in long bursts. Ten seconds at a time, then thirty seconds’ rest. Starter motors overheat, and on a petrol engine continued cranking floods the cylinders and washes raw fuel into the catalytic converter.
• Do not jump-start carelessly. Modern cars often have designated jump points rather than the battery itself, and many have a current sensor on the negative terminal that must not be bypassed. Two minutes with the handbook is cheaper than an ECU.
• Do not bump-start an automatic (it will not work) or anything with a catalytic converter and a full fuel system, unless you have no alternative.
• Do not keep resetting and retrying if the car started and then died. Something is cutting it, and finding out what is cheaper than the outcome.

If it starts after a jump — the important next step

Whether the car keeps running tells you what actually failed. If it starts on jump leads and then dies within a few minutes once the leads are off, it was running on donated power and the alternator itself is not charging. If it starts, runs all day and is flat again the next morning, the battery no longer holds charge or something is draining it while parked.

If the battery warning light stays on once it is running, that settles it — that is a charging fault, not a battery fault.

What does it cost?

When should you call a garage?

Straight away, without cranking again, if you suspect the wrong fuel, if the engine spun over unusually fast and quietly, or if it started and then died repeatedly.

Once you have done the free checks otherwise — headlight test, terminals, spare key, selector position. Those four solve a surprising share of no-starts and none of them costs anything.

When you ring, describe exactly what happens when you turn the key using the table at the top of this page. “Rapid clicking” and “cranks but does not fire” send us down completely different paths, and knowing which saves you money before we start. We do starting and charging diagnostics in Birmingham seven days a week. If there is an engine management light on as well, say so — though note that a car flat enough not to start will often set unrelated fault codes across several systems purely from low voltage, and those are a symptom rather than the cause.