Code 51, Prom Problem
WARNING: This page does not describe the selected car, but rather 22 other vehicles, including the 1992 GMC Yukon, 1992 GMC Vandura Special, 1992 GMC Vandura, 1992 GMC Value Van, and 1992 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.