Fuel System:
* Fuel pump failure: The pump might be weak or failing to deliver fuel, especially if the car has sat for a while. A good initial check is to listen for the pump's whirring sound when the ignition is turned on (it's usually only for a second or two).
* Clogged fuel filter: A restricted fuel filter can prevent sufficient fuel from reaching the engine.
* Fuel pressure regulator problems: This could lead to low fuel pressure.
* Bad fuel injectors: They might be clogged or failing to spray fuel properly. This is less likely to cause a *complete* no-start, but it could be a contributing factor if the problem is intermittent.
* Fuel line leaks or restrictions: A leak or blockage anywhere in the fuel line will prevent fuel delivery.
Ignition System (beyond the computer's control):
* Distributor cap and rotor problems: Cracks or corrosion in the cap or worn rotor can prevent proper spark distribution.
* Worn spark plugs or wires: These can cause misfires or a lack of spark entirely. Inspect for excessive wear, carbon fouling, or damage.
* Ignition coil failure: This supplies the high voltage for the spark plugs. A faulty coil will result in no spark.
* Crankshaft position sensor (CKP): While often throwing a code, a failing CKP can sometimes produce intermittent or subtle failures that don't register. The engine cranks, but the ECU doesn't get the signal to fire the injectors. (though usually this would throw some code, even an intermittent one)
Other Possibilities:
* Security system issues: Some older anti-theft systems can prevent starting even if the engine cranks. Check your car's manual or the system’s instructions.
* Neutral safety switch: This prevents starting unless the transmission is in Park or Neutral. A faulty switch will prevent starting.
* Starter motor issues: While you say the engine rotates, it might be the starter motor's gear isn't fully engaging. However, if it's truly rotating the engine, this is less likely.
* Low battery voltage: While it may crank slowly, even a weak battery can prevent the fuel pump from getting enough power to prime or the spark to jump properly.
* Timing belt (or chain) failure: A broken timing belt will mean the engine will crank, but won't fire. However, you usually hear a different sound and this is extremely unlikely to happen without making noise.
Troubleshooting Steps:
1. Check for fuel delivery: This is usually the first thing to check. Listen for the fuel pump. You might even need to use a fuel pressure gauge.
2. Inspect spark plugs and wires: Check for cracks, damage, or fouling.
3. Check the distributor cap and rotor: Look for cracks or corrosion.
4. Test the ignition coil: This usually requires a multimeter.
5. Check the battery voltage and connections.
6. Verify the neutral safety switch is functioning correctly.
It's crucial to work systematically through these possibilities. If you are not comfortable working on your car's electrical system, it's best to take it to a mechanic. Some of these issues can be tricky to diagnose even with a scan tool.
