* Dead Battery (or weak battery/alternator): Even if the engine cranks, a weak battery might not provide enough power to consistently spark the plugs or run the fuel pump. Test the battery voltage and have the charging system checked.
* Bad Starter Motor (unlikely if it's cranking): While a failing starter *can* sometimes crank weakly, if it's turning over strongly, the starter motor itself is less likely to be the problem.
* Faulty Ignition System: This encompasses several components:
* Spark Plugs: Worn, fouled (covered in oil or carbon), or improperly gapped spark plugs prevent ignition.
* Ignition Coils: These supply the high voltage needed for sparking. A faulty coil can prevent one or more cylinders from firing.
* Ignition Wires (or Plug Wires): Damaged or loose wires can interrupt the spark's path to the plugs.
* Crankshaft Position Sensor (CKP) or Camshaft Position Sensor (CMP): These sensors tell the computer the engine's rotational position, crucial for precise ignition timing. A faulty sensor prevents proper spark delivery.
* Ignition Control Module (ICM): This module controls the ignition system. A failure here can prevent sparking altogether.
* Fuel Delivery Problems:
* Empty Fuel Tank (obvious, but worth checking!): Make sure you actually have fuel.
* Fuel Pump: A faulty fuel pump can't deliver fuel to the engine. You might hear a faint whine from the pump when you turn the key; lack of whine suggests a problem.
* Fuel Filter: A clogged fuel filter restricts fuel flow.
* Fuel Injectors: Clogged or faulty injectors prevent fuel from being sprayed into the cylinders.
* Fuel Pressure Regulator: This regulates fuel pressure; a malfunction can lead to insufficient pressure.
* Security System Issues: Some vehicles have immobilizers or alarm systems that can prevent starting if they detect a problem. Check your owner's manual for troubleshooting these systems.
* Other Sensors: Various sensors (like the Mass Air Flow sensor or Throttle Position Sensor) provide crucial information to the engine control unit (ECU). A faulty sensor can lead to incorrect fuel/air mixture calculations and prevent starting.
Troubleshooting Steps:
1. Check the obvious: Fuel level, battery voltage.
2. Listen carefully: Pay attention to any unusual sounds during cranking (e.g., clicking, whining).
3. Check for error codes: Use an OBD-II code reader to diagnose any trouble codes stored in the ECU. This can pinpoint the problem area.
4. Visual inspection: Check for any visible damage to wiring, hoses, or components.
If you're not comfortable working on your car yourself, take it to a mechanic for diagnosis and repair. Describing the symptoms (cranks but doesn't start) will help them quickly pinpoint the problem.
