If your vehicle is showing car won't start, just makes a rapid clicking noise, here are the most likely causes and the parts you'll need to fix each one.
The battery lacks enough power to spin the starter motor. The rapid clicking is the starter solenoid repeatedly engaging and disengaging as the voltage drops under load.
Corrosion or loose clamps create high electrical resistance, preventing the starter from getting enough current even if the battery is good. This is a frequent installation error if terminals aren't fully tightened after a recent battery replacement.
The alternator charges the battery while driving. If it fails, the car runs entirely on battery power until it dies, resulting in a dead battery and the rapid clicking symptom the next time you try to start it.
The starter solenoid's internal electrical contacts can burn or wear out over time. When this happens, the solenoid may rapidly click instead of transferring full power to spin the starter motor.
A loose or damaged ground strap between the engine block and the chassis restricts electrical flow, mimicking a dead battery. This often occurs if ground wires are left loose or unattached after recent engine or transmission work.