If your vehicle is showing intermittent starting issues (starts perfectly sometimes, refuses to start others), here are the most likely causes and the parts you'll need to fix each one.
Poor contact at the battery terminals can cause intermittent power loss to the starter. This is highly common after a recent battery replacement if the terminals weren't tightened properly or if corrosion has built up.
The starter motor develops 'dead spots' as its internal brushes wear out. Depending on exactly where the internal motor stops spinning, it may start perfectly one time and just click the next.
Relays are electrical switches that can intermittently stick open or closed due to internal wear or heat, preventing the starter signal from reaching the motor.
If the engine cranks normally but refuses to fire, the fuel pump may be intermittently failing to prime the system. This is often worsened by heat or driving with low fuel levels.
This switch verifies the car is in Park/Neutral or the clutch is pressed. If the contacts wear out, the car may refuse to crank unless you jiggle the shifter or repeatedly stomp the pedal.
This sensor tells the engine computer when to fire the spark plugs and fuel injectors. It often fails intermittently when hot, causing a crank-but-no-start condition until the engine cools down.
The vehicle's anti-theft system may intermittently fail to recognize the security chip in your key or fob, actively cutting off the fuel or starter system to prevent theft.