How a car starts, step by step

When you insert the key into the ignition lock and turn the ignition ON, you move the contacts in the Ignition Switch located inside the ignition lock. A bad or worn out ignition switch is one of the common reasons for a car not to start. Sometimes, if the car won't start and jiggling the key in the lock helps to start it, the ignition switch could be bad.

When the ignition switch is in ON position, it sends the 12-volt power to the vehicle's Main Relay that powers the engine computer or as it's usually called, the Engine Control Module or ECM. The Main Relay is usually located in the fuse or relay box (in the photo). If the main relay is bad, which is not uncommon, the engine computer won't receive any power and the car won't start.

The Check Engine Light comes on in the instrument panel to let you know that the engine computer is powered. If the Check Engine light doesn't come on with the ignition ON, it means that the engine computer is not powered and the car won't start.

At the same time, a sensor for the Security System reads the code from the ignition key. This sensor is usually located near or inside the ignition lock. This works the same way as when you place an access card to a reader to open the door. If the code is not correct, or if there is a problem with the security system itself, the sensor will send the signal to the engine computer that the engine is not allowed to start. In this case, you will probably see a "Security" or a key-shaped light blinking on your instrument panel.
If the ignition key code is correct, the security system sends the signal to the engine computer that the car is allowed to start and the security light will go off.
Often a problem with the security sensor or its wiring can cause a car not to start. This was a fairly common issue with some GM vehicles.

Now, when you turn the ignition key further to the "START" position, the ignition switch directs the electric power through the Neutral Safety Switch to the starter relay. A Neutral Safety Switch is needed to prevent a car from being started unless the automatic transmission is in Park or Neutral position. If the automatic transmission is not in Park or Neutral position, the neutral safety switch is open.
Similarly, if a car has a manual transmission, a neutral safety switch disables the starter motor unless the clutch pedal is depressed.

In a car with an automatic transmission, the neutral safety switch could be located in the transmission shifter mechanism inside the car or the Transmission Range Switch (in the photo) can be used as a neutral safety switch. The transmission range switch is usually installed on a gear selector shaft of the automatic transmission. If a car with an automatic transmission doesn't start in Park position, but starts in Neutral, there might a problem with a neutral safety switch or with the shifter mechanism.

If the automatic transmission is in Park or Neutral position or in a vehicle with a manual transmission, the clutch pedal is depressed, the neutral safety switch closes the circuit, so the power goes from the ignition switch through the neutral safety switch and to the Starter Relay (in the photo).

The starter relay powers the Starter Solenoid. The starter solenoid can be installed on a starter motor (in the photo) or as a separate unit. The starter solenoid usually has one small connector (the control wire from the starter relay) and two large terminals: one is connected to the positive battery cable and the other goes to the starter motor. The 12V power sent from the starter relay engages the starter solenoid, which connects the battery power to the Starter Motor. At the same time, the starter solenoid pushes a small starter motor gear forward, so it meshes with the teeth on the engine flywheel and the starter motor starts to turn over the engine.

Once powered, the starter motor makes that cranking sound that you usually hear when starting a car.
If your car won't start, it's important to know whether the engine is being turned over by the starter or not. If the starter does not crank, the problem is within the starting system (the steps above). Sometimes, if the small starter gear for some reason fails to mesh with the engine flywheel teeth, it can make a very loud screeching or grinding noise and the engine won't be turned over.

When the starter motor cranks the engine, the crankshaft and the camshaft(s) start turning. The camshaft(s) is connected to the crankshaft via the timing belt, chain or gears. See the animated diagram here: DOHC engine.
The Crankshaft Sensor, or as it often called, the Engine Speed Sensor (in the photo) and the Camshaft Position Sensor send the signals to the engine computer.

The engine computer uses the signals from the crankshaft and camshaft position sensors to know which cylinder is in the compression cycle. If either of the sensors is bad, the car won't start. Similarly, if the timing belt, chain or gears are broken, the camshaft won't turn and the engine won't start.

Based on the signals from the crankshaft and camshaft position sensors as well as some other sensors, the engine computer powers the Ignition System sending a very high-voltage impulses in appropriate times to the spark plugs.

At the same time, the engine computer powers the Fuel Pump Relay that sends the electric power to the Fuel Pump. The fuel pump relay is also one of the common components to fail.

The fuel pump starts running, building up the fuel pressure in the fuel injector rail. The engine computer alternatively opens electronic fuel injectors that spray short bursts of gasoline vapors into the intake manifold (or in case of the direct injection, straight into the cylinders). If the fuel pump is bad, the car won't start.

As the starter motor turns the engine over, the pistons move up and down in the cylinders building up the compression in the combustion chambers. The sparks produced by the spark plugs ignite the gasoline vapors and the engine starts running on its own. If there is not enough compression in the cylinders, the car won't start.