Car Cranks but Won't Start and How to Find the Cause

You turn the key, the starter spins the engine over strongly, and nothing happens. When a car cranks but won't start, the battery and starter are usually doing their job, which means the fault is somewhere in the three things an engine needs to run: fuel, spark and air at the right moment.

The trick is to test those in a sensible order instead of guessing and buying parts. Below is the sequence I use in the driveway, with the tools most people already own and the signs that point one way or the other.

First, confirm it really cranks but won't start

A lot of "no start" calls turn out to be slow cranking. That is a battery or starter problem, not a running problem, and it needs a different approach. A healthy crank sounds even and quick, roughly 200 rpm or more. A weak crank sounds laboured, slows with each turn, or clicks.

If you have a multimeter, check battery voltage while a helper cranks. It should stay above about 9.6 volts. If it drops below that, deal with the battery first, because a weak crank can also starve the engine computer and fuel pump of voltage, which produces a crank with no start that looks like something else entirely.

Also check the obvious items that catch experienced people out:

Fuel gauge reading. Gauges fail, and a car "with a quarter tank" sometimes has none.
Security light. A flashing key or padlock symbol on the dash often means the immobiliser has not recognised the key.
Recent events. A cheap fill-up, a car wash, a flat battery jump or a battery swap all narrow down the list.

Read the codes before touching anything

Plug in an OBD2 scanner, even with the engine not running. Codes stored during the failed starts are often the fastest answer. Our guide on how to read OBD2 codes at home covers the basics if you have not used one before.

Codes that matter most for a crank with no start:

Code	Meaning	Why it stops the engine
P0335	Crankshaft position sensor circuit	Without a crank signal, most engines fire no spark and no injectors
P0340	Camshaft position sensor circuit	Some engines will not start without cam timing
P0230	Fuel pump primary circuit	Pump relay or wiring fault cuts fuel delivery
P0087	Fuel rail pressure too low	Weak pump, blocked filter or failed regulator
B or U immobiliser codes	Key not recognised	Computer blocks injection on purpose

One useful scanner trick: watch live data for engine rpm while someone cranks. If rpm reads zero during cranking, the computer is not seeing the crankshaft sensor, and that sensor or its wiring is your prime suspect.

Check for fuel

Fuel is the most common cause of a crank with no start, and the first test costs nothing.

Turn the key to "on" without cranking. Listen near the fuel filler or rear seat for a two to three second hum. That is the pump priming.
No hum? Check the fuel pump fuse and swap the pump relay with an identical one from the fuse box, such as the horn relay. Try again.
If you hear the pump, check pressure. Many port injected engines run 40 to 60 psi. A fuel pressure gauge kit costs 30 to 60 dollars and screws onto the Schrader valve on the rail if your engine has one.
Pressure that drops quickly after the pump stops suggests a leaking injector, regulator or check valve.

On direct injection engines, rail pressure is far higher and the test valve may not exist. Use the scanner's fuel pressure readings instead and do not open those lines.

Check for spark

If fuel looks good, move to ignition. An inline spark tester costs about 10 dollars and is much safer than holding a plug against the engine.

Pull one coil or plug lead and connect the tester between the coil and the plug.
Have a helper crank for three to five seconds.
A bright, regular flash means that coil is firing. No flash on any cylinder points to a shared cause like the crank sensor, a main ignition fuse or the engine computer.
No flash on one cylinder only will not stop the engine starting. It causes a misfire instead, which our guide to diagnosing a misfire without a scanner covers.

Pull a spark plug while you are there. A plug wet with fuel after cranking means fuel is arriving but not lighting, which points back toward spark or a flooded engine.

Why a starting spray is not the first test

The classic advice is to spray starting fluid into the intake. If the engine fires briefly, the problem is fuel. If it does not, it is spark. It is quick and it often works, which is why so many people swear by it.

I rarely reach for it first, for three reasons. Starting fluid is ether, and on modern engines with plastic intake manifolds a backfire can crack the plenum, turning a 20 dollar relay fix into a 400 dollar part. Diesel engines with glow plugs or intake heaters can be damaged badly by it. And the result is less clear than people think, because an engine with weak spark may still pop on ether while refusing to run on petrol.

The fuel pump hum, the relay swap and an inline spark tester answer the same question with no risk. Keep the spray as a last confirmation on older engines, and use a tiny amount if you do.

Less obvious causes worth knowing

When fuel and spark both check out, look at these:

Flooding. Repeated short cranks can soak the plugs. Many engines have a "clear flood" mode: hold the accelerator to the floor while cranking, which cuts injection.
Timing belt or chain. A broken belt makes cranking sound faster and more even than normal, because there is no compression. Stop cranking immediately on an interference engine.
Bad fuel. Water in the tank or the wrong fuel after a fill-up will crank with no start or start and die.
Inertia switch. Some Ford and older European cars have a fuel cut-off switch that trips after a bump. It is usually in a footwell or boot and has a reset button.

If none of these find the fault, the remaining suspects are wiring, the engine computer and the immobiliser module. That is the point where a shop with a proper scan tool and wiring diagrams earns its fee.

Frequently asked questions

What is the most common reason a car cranks but won't start?

Fuel delivery is the most common cause, usually a failed fuel pump, pump relay or blown fuse. A failed crankshaft position sensor is a close second on many modern cars. Listening for the pump prime and checking codes will separate the two in a few minutes.

Can a bad battery cause a crank with no start?

Yes. A battery that cranks slowly can drop voltage below what the engine computer, injectors and fuel pump need to work. Check that voltage stays above about 9.6 volts during cranking before you chase fuel or spark faults.

Can a car crank but not start because of the key?

It can. If the immobiliser does not recognise the transponder chip in the key, the engine computer blocks fuel injection while still letting the starter turn. Try the spare key and watch for a flashing security light on the dash.