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Forum > Failed Smog Test > Question

2001 GMC Sierra Pickup - Recently Check Engine Light Came On Due to a By-Pass Exhuast Valve.

We replaced the valve and reset the check engine light. However, smog test shows monitors incomplete. Technician says I have to run the truck 20 or 30 miles in order for the computer to capture all the emission components working. I drove for 30 miles, got a retest and still my GMC did not pass. Emission results are great and everything else passed. What is going on, I can't be paying $30 for a rechecked anymore.
Answer:

Your GMC Sierra is equipped with an On-Board Diagnostics (OBD II). The OBD II system monitors all aspects of your GMC's emissions system and reports information to the OBDII computer. The data is processed and checked against the computer's pre-determined values for various inputs levels and performance patterns of your vehicle. If any problems are found the computer will determine whether to alert the driver or not. It appears during your recent repair of the exhaust valve your trucks OBD II computer system was reset; erasing all the emission monitors (readiness flags) from the OBD II computer. Setting the emission monitors requires driving your truck at least 200 miles; consisting of everyday normal driving, stop and starts (like at red lights and traffic), ignition on and off several times a day with at least 15 minute causes in between (this allows the EVAP monitor to run), and freeway driving at least on three occasions for at least 15 minutes each at speeds above 30 mph. After these driving conditions are met and your GMC still fails the OBD II portion of the smog check, we can only recommend a smog check repair center conduct a diagnosis of the OBD II computer as to clues of why the monitors are not setting. Assuming no Check Engine light or Malfunction Indicator Lamp is illuminated we have to suspect your vehicle's only problem is simply not enough drive time yet. : As far as spending $30 for a retest on and on... We can only recommend you build a good relationship with your smog station, promise you will send them all your business and recommend them to all your family and friends, and ask that they simply connect their scan tool to your truck only to inspect if all the monitors have set; prior to doing an actual smog test. Plugging in the scan tool and retrieving monitor data is fairly easy to do. posted by SmogTips Support

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