Your best bet is going to be with a dealer or smog check repair station diagnosis. Thousands of miles of driving and with having followed the Chevy Silverado drive cycle, and still not ready OBD-II monitors will require some hands-on investigation.
A repair shop will need to put your truck on a dynomometer and begin scanning the ECM as the vehicle is being driven through its drive cycle. They will be able to detect exactly where and when the drive cycle is failing, and why certain monitors are unable to complete.
posted by SmogTips Support