Is it safe to keep driving?
The car is not going anywhere under its own power. Avoid cranking repeatedly in long bursts — you will flatten the battery and, on some engines, wash oil off the bores with unburnt fuel. Crank in short attempts and diagnose between them.
General guidance, not a diagnosis of your specific vehicle. If you are unsure, do not drive it and have it inspected.
Start with the timing reference
Before the ECU will fire an injector or a coil it needs to know where the crankshaft is. If the crankshaft position sensor signal is missing or implausible, most engines will crank indefinitely with no spark and no injection, and often with no engine-speed reading on the scan tool at all. That last detail is the fastest test available: if RPM stays at zero while the engine is physically turning, the ECU cannot see the engine turning.
Spark, fuel, compression — in that order of ease
Fuel is usually the easiest to confirm: turn the key to on without cranking and listen for the two-second pump prime from the rear of the car. Spark and fuel pressure both need a little equipment, but a scan tool showing commanded injector pulse width while cranking tells you the ECU is at least trying. Compression is last because it is the most work, and a sudden total no-start from a timing belt or chain failure usually announces itself with an unusually fast, smooth, hollow-sounding crank.
Security and immobiliser
If the engine cranks but the immobiliser has not authorised the key, many vehicles cut injection entirely. A security indicator that stays lit or flashes while cranking is the tell. This is not an emissions fault and often stores no powertrain code at all.
Flooded, or simply out of fuel
Repeated failed starts can flood the engine. Many engines have a clear-flood mode: hold the throttle fully open while cranking and the ECU cuts injection to let the cylinders clear. And verify fuel level independently of the gauge — a failed level sender reads whatever it feels like.
What to watch in live data
These are the readings that separate the likely causes. OBDAssistant streams them from an ELM327 adapter while the engine is running, which is what turns a list of possibilities into a diagnosis.
| Reading | What it tells you |
|---|---|
| Engine RPM while cranking | The single most valuable reading. Zero RPM during a physical crank means the crank or cam signal is not reaching the ECU. |
| Crankshaft and camshaft position sensor status | A correlation fault between the two will prevent starting even when both sensors individually appear alive. |
| Fuel rail pressure | Confirms whether the high-pressure side is actually building pressure, not just whether the pump hums. |
| Injector pulse width | If the ECU is commanding injection and the engine still will not fire, the problem is downstream of the ECU. |
Trouble codes that match this symptom
If the car has already been scanned, these are the codes most often behind it. Each page covers the definition, likely causes, diagnostic steps and repair information.
- P0335 Crankshaft Position Sensor A Circuit Malfunction
- P0340 Camshaft Position Sensor A Circuit Malfunction (Bank 1)
- P0341 Camshaft Position Sensor A Circuit Range/Performance
- P0016 Crankshaft Position - Camshaft Position Correlation (Bank 1 Sensor A)
- P0087 Fuel Rail/System Pressure - Too Low
- P0201 Injector Circuit Malfunction - Cylinder 1
- P0202 Injector Circuit Malfunction - Cylinder 2
- P0203 Injector Circuit Malfunction - Cylinder 3
- P0601 Internal Control Module Memory Check Sum Error
- P0606 ECM/PCM Processor Fault
- U0100 Lost Communication With ECM/PCM "A"
Background reading
Questions
It started after a few more tries. Is it fixed?
No. An intermittent crank or cam sensor, a failing fuel pump, or a marginal immobiliser antenna all behave exactly like this before they fail completely. Scan it while the fault is fresh — stored codes and freeze-frame data are far more useful than a memory of what happened.
Could a flat battery cause this if it still cranks?
Yes. Cranking draws enough current to pull system voltage down, and if it sags far enough the ECU may brown out or the fuel pump may not reach pressure. A battery can turn the starter convincingly and still be unable to support the rest of the system.
There are no codes at all. What does that mean?
Often that the ECU never ran a diagnostic cycle, because it never saw the engine turn. A total absence of codes on a cranking no-start points toward the timing reference or power supply to the ECU rather than away from them.