* Dead or weak battery: This is the most common cause. A battery that can't deliver enough current will cause the starter motor to crank slowly or not at all.
* Faulty starter motor: The starter motor itself might be worn out, damaged, or have internal problems preventing it from engaging the flywheel. This could be due to worn brushes, a bad solenoid, or other internal failures.
* Bad starter solenoid: The solenoid is the electromagnetic switch that activates the starter motor. A faulty solenoid won't engage the starter, even if the battery and motor are good.
* Poor connections: Corrosion or loose connections in the battery cables, starter motor cables, or the solenoid wiring can prevent sufficient current from reaching the starter.
* Faulty ignition switch: A problem with the ignition switch could prevent power from reaching the starter.
* Neutral safety switch (if automatic transmission): This switch prevents starting unless the transmission is in Park or Neutral. A malfunctioning switch could prevent starting.
* Low compression: While not directly related to the *starting* system, low compression in the engine will make cranking harder. The starter might struggle, or fail to turn the engine over, if the engine's internal pressure is too low.
To troubleshoot, a mechanic would likely check the battery voltage, test the starter motor, inspect the wiring and connections, and potentially test the compression if other checks are inconclusive.
