Engine Backfires

Ensure proper spark exists from ignition system.
Ensure ignition timing is correct.
Check valve timing and timing belt (if equipped).
Ensure fuel system performs properly (fuel pump, fuel filter, fuel injector and fuel pressure regulator).
Check EGR system.
Check Throttle Position Sensor (TPS) and wiring.
Check airflow meter or vacuum sensor and wiring.
Check coolant temperature sensor and wiring.
Check air temperature sensor and wiring.
Check oxygen sensor and wiring.