Diagnostic Aids
WARNING: This page is about a different car, the 2002 GMC Cab & Chassis Sierra and 2002 Chevrolet Cab & Chassis Silverado. However, it is still accessible from the selected car via links, so may be relevant.
First, diagnose and clear any engine DTCs that are present. Then inspect for any transmission DTCs that may have reset. If a DTC P0717 and a DTC P0730 are both present, diagnose DTC P0717 first.
Sticking or contaminated shift valves may cause an undefined gear ratio.
DTC P1870 detects an incorrect gear ratio in 4th gear with TCC applied.
DTC P0730 defaults to an elevated line pressure condition which may result in TCC partial apply. This may induce idle surge and engine stall.