Your Chevy Duramax will need to be driven at least 200 to 300 miles through various driving conditions in order to complete Emission Monitors (also known as readiness flags); which are required to pass the smog check.
Emission monitors are internal self-tests which the Engine Control Unit performs to ensure a vehicle is not polluting and is running efficiently. The entire process of self-testing various emission control systems is called a Drive Cycle. A diesel engine drive cycle is similar to that of a gasoline engine.
How to Perform Chevy Duramax Drive Cycle
The following is a Chevrolet OBDII Drive Cycle Pattern. (Obey all traffic laws while performing drive cycles. Perform acceleration steps only on highways. If you feel you will be unable to perform the drive cycle safely, seek assistance from a Chevy dealership or smog check test & repair center).
COLD START: Begin from Cold Start, and ignition in OFF position for at least 1 hour. Ensure fuel tank is between 1/4 and 3/4 full. Engine coolant temperature must be below 50°C (122°F) and within 6°C (11°F) of the ambient air temperature. Do not leave the key on prior to the cold start or the heated oxygen sensor diagnostic may not run.
IDLE TIME: Run engine (do not drive) for 2 1/2 minutes with the air conditioner on and rear defroster on. During this period the ECU will be running the Oxygen Sensor Heater self-test, Air Injection System Passive Air test, EVAP Purge "No Flow" test, Misfire Detection.
ACCELERATE: Turn off the air conditioner and rear defroster and begin driving. As soon as possible apply half throttle until 88km/hr (55mph) is reached. The ECU will now run self-tests on the Misfire Detection, Fuel Trim, and EVAP Purge Flow.
HOLD STEADY: Now hold your speed steady at 88km/hr (55mph) for three minutes. During this time the Oxygen Sensor O2 response, Air Intrusive (AIS System), EGR Flow, Purge Valve, Misfire Detection, and Fuel Trim diagnostics will be performed.
DECELERATE: During this step you want to slowly let off the accelerator pedal. Do not brake or shift in order to slow down (don't press the clutch if equipped). It is important to let the vehicle coast along gradually slowing down to 32km/hr (20 mph). The ECU will once again self-test the EGR System, EVAP Purge System and Long Term Fuel Trims.
ACCELERATE: Go ahead and accelerate again at 3/4 throttle until 88-96 km/hr (55-60mph) is reached. This will perform the same self tests as during the first acceleration step.
HOLD STEADY: Hold steady speed once again. This time at 88km/hr (55mph) for five minutes. During this time, in addition to running the self-tests as during the first HOLD STEADY step, the ECU will perform the catalyst monitor diagnostics. If the catalyst (Catalytic Converter) is marginal or the battery has been disconnected, it may take 5 complete driving cycles to determine the state of the catalyst,or in order to Complete the CAT Monitor.
DECELERATE: This will perform the same diagnostics as in the first Deceleration. Remember not to press the brakes or shift gears (don't press the clutch if equipped).
This completes the Chevy Duramax drive cycle, and should make Complete/Ready all required Emission Monitors requried to pass the smog test. If you have access to an OBDII scan tool you can check the emission monitors for READY status before revisiting the smog check center.
