Code 51, Prom Problem
WARNING: This page does not describe the selected car, but rather 22 other vehicles, including the 1990 GMC Vandura Special, 1990 GMC Vandura, 1990 GMC Value Van, 1990 GMC Suburban, and 1990 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 okay, replace PROM and recheck. If problem is not corrected, replace ECM.