DTC 51, Prom Problem
WARNING: This page does not describe the selected car, but rather 20 other vehicles, including the 1993 GMC Yukon, 1993 GMC Vandura Special, 1993 GMC Vandura, 1993 GMC Suburban, and 1993 GMC Rally Camper Special. However, it is still accessible from the selected car via links, so may be relevant.
Ensure all pins are fully inserted in socket. If pins are fully inserted, replace PROM and recheck. If problem is not corrected, replace ECM.