The answer is simple:

Buy the o2 that matches the year car the ECU came from. If youre using an OBD1 ECU, buy the o2 from the OBD1 car. If youre using the 96+ exhaust, you will need to block off the REAR o2 since the OBD1 ecu won't monitor the catalytic converter and won't need the rear 02.

Also, if you've somehow got the obd2 o2 sensor wired in, that can cause the problem. When doing year/obd conversions, use the harness & sensors that MATCH the ECU.