DTC 55: ECM/Pcm Error
WARNING: This page does not describe the selected car, but rather 19 other vehicles, including the 1996 Chevrolet Chevy Van, 1995 GMC Yukon, 1995 GMC Vandura Special, 1995 GMC Vandura, and 1995 GMC Suburban. However, it is still accessible from the selected car via links, so may be relevant.
Ensure control module grounds are good and MEM-CAL is properly latched. If okay, replace control module. Clear codes and confirm closed loop operation. Check operation of SERVICE ENGINE SOON light.