GM Service Manual Online
For 1990-2009 cars only

Checks

Causes

Check and adjust the select linkage

Improper adjustment of select linkage-- Refer to Park/Neutral Position Switch Replacement

Check adjustment of shift switch

Improper adjustment of shift switch-- Refer to Park/Neutral Position Switch Replacement

Transmission control module (TCM)

Refer to Diagnostic System Check - Vehicle.

Engine mechanical or electrical

Refer to Diagnostic Starting Point - Engine Controls.