I/M Failure Diagnosis
What the inspector is checking
The I/M program verifies the MIL commands on at key-on, that no DTCs are stored, that all supported monitors are complete, and, depending on the area, that tailpipe gases or a gas cap test pass. Your job is to fix the underlying fault and get the monitors to run.
Readiness monitors
Not Ready is the most common failure, and it usually follows a battery disconnect, a dead battery, or a scan-tool code clear. Monitors need specific enabling conditions. The EVAP monitor typically wants a cold soak, a fuel level between about 15% and 85%, and a steady cruise; catalyst and O2 monitors want a warm engine at cruise speed. Drive cycles vary by manufacturer, so look them up and follow them exactly.
Permanent codes
Permanent DTCs cannot be erased with a scan tool. They clear only after the vehicle runs the monitor and passes it. Clearing codes to get through a test backfires: the MIL goes out, monitors go incomplete, and the permanent code remains.
Traps
Clearing codes to help the customer; replacing a PCM because the MIL bulb was burned out; a gas cap that seals on the car but fails on the tester; and applying a generic drive cycle to a vehicle that needs a specific one.
Rule of thumb
Never clear codes as a repair. Fix the fault, run the manufacturer's readiness drive cycle, and confirm with the scan tool that monitors are complete and no permanent DTC remains before sending the vehicle to the test station.