Code 55, ECM/Pcm Error
WARNING: This page does not describe the selected car, but rather 18 other vehicles, including the 1994 GMC Vandura Special, 1994 GMC Vandura, 1994 GMC Suburban, 1994 GMC Rally Camper Special, and 1994 GMC Rally. However, it is still accessible from the selected car via links, so may be relevant.
Ensure ECM grounds are good and MEM-CAL is properly latched. If okay, replace ECM/PCM. Clear codes and confirm closed loop operation. Check operation of SERVICE ENGINE SOON light.