Code 51, Prom Problem
WARNING: This page does not describe the selected car, but rather 22 other vehicles, including the 1991 GMC Vandura Special, 1991 GMC Vandura, 1991 GMC Value Van, 1991 GMC Suburban, and 1991 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.