If the engine cranks — the starter spins it over — but it will not fire up, the starting system is fine. The problem is one of the three things an engine needs to run: spark, fuel, or compression. This guide works through them in order so you can find why your car cranks but won’t start.
Cranks vs won’t crank
This guide is for an engine that turns over but will not catch. If instead you get a single click, rapid clicking, or nothing when you turn the key, that is a no-crank problem (usually battery, starter, or connections) — use the car won’t start diagnosis for that case.
Spark, fuel, or compression
| Need | Common failures | Quick check |
|---|---|---|
| Spark | Crankshaft or camshaft position sensor, ignition coil/pack, immobilizer not recognizing the key | Check for spark with an inline tester; see if the immobilizer light stays on |
| Fuel | Failed fuel pump, blocked filter, bad gas or water in fuel, no fuel pressure | Listen for the pump prime; check fuel pressure |
| Compression / timing | Snapped timing belt, jumped timing | Cranks unusually fast and smooth = possible broken belt |
Step-by-step: find why it won’t fire
- Note how it cranks. Normal cranking speed is good; unusually fast, smooth cranking can mean lost compression from a broken timing belt.
- Check the immobilizer: if a key/security light stays lit or flashes, the car may not be recognizing the key, which blocks spark and fuel. Try the spare key.
- Test for spark: an inline spark tester (clipped between coil and plug) is the safe, reliable way — crank and look for a strong spark. No spark points to the crankshaft or camshaft position sensor, the coil, or the immobilizer.
- Test for fuel: turn the ignition to “on” and listen for the fuel pump priming for a couple of seconds. No prime, or no fuel pressure, points to the pump, relay, or fuse.
- Consider recent fuel: if it started right after refueling, bad gas or water in the tank can stop it firing.
- If spark and fuel are present and timing is intact but it still will not start, the fault is deeper (compression/timing) and needs a workshop.
The most common causes
On a car that cranks but won’t start, the usual suspects are a failed crankshaft or camshaft position sensor (no spark signal), a fuel pump that has quit, or an immobilizer not reading the key. A snapped timing belt is less common but serious — on an interference engine it can bend valves as it goes, so it is not only a no-start; suspect it if the belt is due or overdue, especially with fast, smooth cranking.
Frequently asked questions
Why does my car crank but not start?
Because cranking proves the starter and battery are working, the missing ingredient is spark, fuel, or compression. Most often it is a crank position sensor, the fuel pump, the immobilizer, or — less commonly — a broken timing belt.
How do I know if it is fuel or spark?
Listen for the fuel pump priming when you switch the ignition on, and test for spark at a plug while cranking. Whichever is missing tells you which system to chase first.
Can a dead key fob stop the car starting?
A flat fob battery usually still starts the car if you hold it near the start button or use the physical key, but an immobilizer fault that does not recognize the key will prevent starting. Try the spare key.
Could bad fuel stop it starting?
Yes. A bad tank of fuel or water in the fuel can prevent the engine firing. If the problem began right after a fill-up, that is a strong clue.
