Code 51, Prom Error (Faulty Or Incorrect Prom)
WARNING: This page is about a different car, the 1994 GMC Sonoma and 1994 Chevrolet S10 Pickup. 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.