Code EO51, Prom Error
WARNING: This page is about a different car, the 1986 Oldsmobile Toronado and 1986 Buick Riviera. However, it is still accessible from the selected car via links, so may be relevant.
Check that all pins are fully inserted in the socket. If okay, replace PROM, clear memory, and recheck. If Code 51 reappears, replace ECM.