my 85 runner is throwing trouble codes #21 oxygen sensor - detection of sensor/signal deteriotion or open/short in sensor heater signal.. #51 switch signal - IDL,NSW, or A/C signal to ECU missing with check terminals E1 and T shorted... #71 EGR system malfunction - EGR gas temperature below limit during EGR operation, my chilton doesn't show those codes until 1988. I have replaced my O2 sensor twice with no change the truck seems to run fine but my question is why do I get these codes in my 85

 

I've searched to try to find an answer but haven't had any luck. Any suggestions?