Engine Backfires

Check for vacuum leaks.
Check idle speed and ensure ignition timing is correct.
Check cold start injector and cold start injector time switch.
Ensure fuel system (pump, lines, filter and pressure regulator) performs properly.
Check for clogged fuel injectors.
Check power to ECU via fusible links, fuses and EFI main relay.
Check airflow meter, coolant temperature sensor, throttle position sensor and air temperature sensor.
Check injection signal circuit.
Check O2 sensor(s).