2002 Oldsmobile Silhouette "leaking antifreeze rear engine 02" Questions

...Antifreeze leak, Heaters don't work. Replaced Water pump and flushed Heater core. Later on Transmission slipping. Replaced 2 sensors. The mechanic checked the codes and they are P0171, P0420, P1537 ...

...rear sensors are part of a passive monitor system. the PCM checks that the CAT is not lazy or dead. using just this sensor. no engine size stated, no DTC number shown. so, no help from me. the sensor ...