* Battery: A weak battery might have enough juice to power the starter motor when it's off the car, but not enough to crank the engine under load. Test the battery voltage with a multimeter. It should be around 12.6 volts when fully charged. A load test is even better; it simulates the cranking load.
* Battery Cables: Check the battery terminals and cables for corrosion, loose connections, or damage. Clean the terminals thoroughly and ensure tight connections. Corrosion can significantly impede current flow.
* Ignition Switch: A faulty ignition switch might not be sending power to the starter solenoid even if the key is turned. This is a less common but possible cause.
* Neutral Safety Switch (NSS): The NSS prevents the car from starting unless it's in Park or Neutral. A faulty NSS will prevent the starter from engaging, even with a good battery and starter.
* Starter Solenoid: While you say the starter *motor* works, the problem might be with the solenoid within the starter assembly. The solenoid is an electromagnet that engages the starter motor's gear. Even if the motor spins when tested separately, the solenoid might be failing to engage the motor when installed. (Sometimes it's hard to test this definitively without putting it back in.)
* Wiring: Inspect all wiring related to the starter, including the positive and negative battery cables, the ignition switch wiring, and the NSS wiring. Look for broken wires, chafing, or poor connections.
* Fuses and Relays: Check the fuses and relays related to the starter motor and ignition system. A blown fuse or faulty relay will cut power to the starter.
Troubleshooting Steps:
1. Battery Test (most important): Start here. A weak battery is the most common cause of this problem.
2. Cable Inspection: Thoroughly inspect and clean battery cables.
3. Neutral Safety Switch Check: Make sure the car is firmly in Park or Neutral (as applicable). If you suspect the NSS, testing it might require a multimeter and a wiring diagram.
4. Visual Inspection of Wiring: Look for any obvious damage to the starter wiring harness.
5. Fuse and Relay Check: Check the relevant fuses and relays in the fuse box(es).
6. Ignition Switch Check (advanced): Testing the ignition switch often requires more specialized tools and knowledge.
If you are not comfortable working on your car's electrical system, it is best to take it to a qualified mechanic for diagnosis and repair. Improperly working on the electrical system can be dangerous.
