Engine Won't Start

Ensure electrical harness, connectors and wires are not broken or loose. Ensure battery voltage is at least 11.5 volts. Check system relay and fuel pump relay. On turbo models, check pressure switch. Check all ECU input signals.
Ensure fuel pump is operating. Check fuel pressure and pressure regulator. Check for leaks or restrictions.
Check injector operation and opening duration. Check for leaks.
Check induction system. Check for air leaks downstream of throttle valve, using pressure tester. Visually inspect between mass airflow meter and throttle housing.
Check coolant temperature sensor operation.
Check auxiliary air valve for leaks and proper operation.