OK, the heating circuit has failed on my downstream O2 sensor(CEL code P0141). As I understand it, this sensor does not affect fuel trim but only monitors exhaust. My Subaru still runs fine with a very slight decrease in MPG. Dealer wants over $300 to replace and says if the sensor is not replaced, damage will occur to catalytic converters. If this is true, please help me understand what is happening. Seems as though the downstream sensor has minimal to no affect on engine performance. Definitely not running in limp mode.