1. 12V Battery is dead – whether due to lights or accessories left on or battery degradation/sulphation over time, the battery lacks the electrical power to turn the engine over. It also takes more to turn the engine over when fluids are cold and viscous like in the winter time.
2. Cam/Crank sensor – on modern vehicles, these sensors tell the computer where the internal engine parts are in relation to each other. If the computer can’t tell (due to a bad sensor), it will not allow the car to start.
3. Out of fuel. Late this morning, got to work with the fuel light on, parked on a hill? The fuel may have drained away from where the fuel pump can pick it up and deliver it to the engine. Occasionally fuel gauges or the senders in the tank that actually read the fuel level can fail too.
4. Battery cables and connections eating up voltage; Even though the battery might be strong enough, corroded (blue/green fuzz) or loose or missing (engine block ground) cables and clamps might be causing “voltage drop”. Ie. Amount of power leaving battery is dropped to below the minimum level before it reaches the starter motor.
5. Starter motor/solenoid worn out. Starter motors have 2 parts; the ignition switch controlled solenoid that controls power and the electrical motor that turns the engine over. The solenoid has full battery power running through it at times and the electrical contacts may burn/wear as a result. The electrical motor itself may also fault resulting in the inability to crank the engine.
6. PRNDL (shifter) switch is out. If you’re driving an automatic car, it only starts in neutral or park by design. Whether the switch has worked itself out of adjustment or had too many lattes spilled on it, the car will not crank if recognized as being in the Park or Neutral conditions.
7. Brake or clutch switch out. For newer vehicles with push button start or a manual transmission vehicle the brake pedal and/or clutch pedal switch must indicate to the computer that the pedal is depressed in order to start the car. As with any mechanical part, after many thousands of cycles, they wear out too.
8. Key fob battery is dead. If you have a vehicle that can start without removing the fob from your pocket, that fob contains a battery which transmits to your vehicle when in close proximity. Should the fob battery die, you vehicle will to recognize you and not start.
9. Ignition switch or key worn out. Every time you drive your vehicle, both your ignition switch and key are worn. Even more if you use the key in the doors/trunk or if you like to relax in your car with just the radio on (ignition in acc. Position). With every use of the key the tumblers in lock cylinders, the electrical contacts in your ignition switch as well as the bumps on your key wear. Eventually they may become unusable.
10. Fuel pump – Considering that the pump runs continuously any time the engine is running (even at idle), it actually more mileage on it than your car has been driving (odometer only counting when wheels turning). Eventually its output diminishes until it can’t supply enough volume/pressure to keep the engine satisfied.