Engine Backfires (Lean Fuel Mixture)

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