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Emissions Component

GMC Silverado/Sierra CAT Monitor Won't Run

Would you guys now why my 2005 GMC Silverado/Sierra 2500hd won't run the catalyst monitor? Even after new catalytic converters and oxygen sensors were installed and computer reset. I can not find anything online. All five other monitors are complete. I'm not sure if I am doing the right drive cycle.


It may several complete drive cycles in order to get the catalyst monitor to run. First off, using a scan tool, make sure that there are no current or pending trouble codes. Next ensure your GMC has between 1/4 to 3/4 fuel in the gas tank, and finally only start the drive cycle from a cold start; start a drive cycle in the mornings, after your Sierra has been sitting overnight. Use extra caution when completing the drive cycle on public roadways. You will need to follow the drive cycle preciously. It may be difficult to maintain certain speeds, decelerate as needed, and come to a complete stop, when driving among other vehicles on city streets and freeways. If you feel you may not be able to perform the GMC drive cycle yourself, please seek assistance from a GMC dealership or experienced smog check repair station.

GMC Silverado/Sierra 2500 Drive Cycle: (Begin Here)

Make sure to start the drive cycle with the engine cold. Engine coolant temperature must be below 122°F at startup. Also ensure fuel is between 1/4 and 3/4, and do not leave the key in the ignition before starting the drive cycle for longer than 5 seconds.

A. Start the engine and allow to idle for 2 1/2 minutes, while the A/C and rear defroster are ON. The drive cycle is now testing the O2 heater, Air Injection, EVAP Purge, and Misfires. 

B. Begin driving. Turn off the A/C and rear defroster and begin to accelerate to 55mph. During this time the drive cycle is checking for misfires, fuel trim, and EVAP purge flow. 

C. Hold your speed steady at 55mph for 3 minutes. The drive cycle is now checking oxygen sensor switch time and response, air injection, EGR flow, EVAP purge, misfires, and fuel trim. 

D. Start slowing down. Begin to decelerate by letting off the gas pedal and without touching the brake (or clutch). Your GMC must slowly coast down to 20 mph. 

E. Speed up. Apply 3/4 throttle accelerate back to 55mph and keep your speed steady for 5 minutes. Along with the same tests being performed as in step C, the catalyst monitor is now running. As mentioned above, it may take several complete drive cycles in order to complete the catalyst monitor portion of the GMC drive cycle. 

F. Start slowing down. Begin to decelerate by letting off the gas pedal and without touching the brake (or clutch). Your GMC must slowly coast down to 0 mph.

The drive cycle is now complete. Using a scan tool check the status of the monitors. If one or more are not yet complete/ready, perform the drive cycle again. Unless the air injection or EVAP monitors are not complete, it will be necessary to start again from a cold start. For all other monitors, start again from Step A, and while your engine is already warm.

posted by SmogTips Support

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