* Ignition Switch: Even with a new lock cylinder, the *ignition switch* itself (the part that actually sends the electrical signal to crank the engine) could be faulty. This is a common failure point. It's possible the switch is only making contact for the accessory circuits (radio, lights, etc.) but not the starter circuit.
* Neutral Safety Switch: This switch prevents the engine from starting unless the transmission is in Park or Neutral. If it's malfunctioning, it won't send the signal to allow the starter to engage. Check your transmission linkage and ensure it's properly adjusted. A bad connection or the switch itself failing are common issues.
* Starter Relay/Solenoid: The starter relay (or solenoid, often integrated) acts as an intermediary, receiving the signal from the ignition switch and then sending a larger current to power the starter motor. A faulty relay will prevent the starter from receiving power. You can test this by directly supplying power (carefully!) to the starter, but doing this incorrectly could be dangerous.
* Starter Motor: The starter motor itself could be failing. It might be seized, have worn brushes, or have internal short circuits. While less likely given the accessories working, it's a possibility.
* Wiring: A broken, corroded, or loose wire in the starting circuit is always a possibility. This could be anywhere between the ignition switch and the starter motor, including the connections to the relay and neutral safety switch. Visually inspect all wiring for damage.
* Battery Cables/Connections: While you mention it runs, weak battery cables or corroded connections can still prevent the starter from receiving sufficient power, even if the accessories work. Clean and tighten these.
Troubleshooting Steps:
1. Check the battery cables and connections: Clean and tighten them thoroughly.
2. Test the battery voltage: Ensure it's at least 12.6 volts when the engine is off. If lower, charging may be necessary.
3. Inspect the wiring: Carefully check all wires related to the ignition switch, neutral safety switch, starter relay/solenoid, and starter motor for damage, breaks, and loose connections.
4. Test the neutral safety switch: Make sure the transmission is in Park or Neutral and that the switch is functioning correctly (this often requires a multimeter).
5. Test the starter relay/solenoid: You can either replace it (relatively inexpensive) or attempt to test it with a multimeter (requires some electrical knowledge).
6. Test the starter motor: This usually involves either removing and bench-testing it, or carefully supplying power directly (be extremely cautious!).
If you're not comfortable working with car electrical systems, it's best to take it to a qualified mechanic. Incorrectly diagnosing or working on electrical components can lead to further damage or injury.
