Code 51, Faulty Prom/Mem-Cal
WARNING: This page does not describe the selected car, but rather 24 other vehicles, including the 1992 GMC Yukon, 1992 GMC Vandura Special, 1992 GMC Vandura, 1992 GMC Value Van, and 1992 GMC Suburban. However, it is still accessible from the selected car via links, so may be relevant.
Ensure all pins are fully inserted in socket. If okay, replace PROM/MEM-CAL, clear memory and recheck. If Code 51 reappears, replace ECM.