Code 55, ECM/Pcm Error
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 ECM grounds are good and MEM-CAL is properly latched. If okay, replace ECM/PCM. Clear codes and confirm closed loop operation. Check operation of SERVICE ENGINE SOON light.