PCM Inhibit

WARNING: This page is about a different car, the 2002 Oldsmobile Aurora. However, it is still accessible from the selected car via links, so may be relevant.

The PCM will inhibit (not allow) cruise control if one or more of the following has occurred:

When Park, Reverse, Neutral, or 1st gear is indicated.
When certain Diagnostic Trouble Codes (DTC) are stored.
When an over/undercharged battery condition exists.
With low engine RPM.
With high engine RPM (Fuel Cut-Off).