Diagnostic Aids
WARNING: This page is about a different car, the 2007 GMC Yukon XL and 2007 Chevrolet Suburban. However, it is still accessible from the selected car via links, so may be relevant.
If the electrical circuit checks are OK and the tow/haul shift pattern is not occurring, there may be a mechanical/hydraulic condition that prevents tow/haul operation. Refer to Symptoms - Automatic Transmission .