GM Service Manual Online
For 1990-2009 cars only

When the ignition is switch to the RUN position the TCM performs a functional check of the solenoid valves, switches and sensors. If a problem is detected at any of the TCM inputs the TCM will enter the limp home mode. In the limp home mode the operator can manually select the gears regardless of vehicle speed. Care should be taken not to downshift and over rev the engine.