got this 0503 code, for the rear bank O2 sensor, circuit fault, what exactly does this mean, i read up on the ECU codes, but im wondering if its the harness or waht, i replaced it with a sensor that is not the same model# rather its the bosch premium version, but the CEL didnt go away which leads me to believe either the harness went bad...or the sensor went bad and replacing it with the 15735 model from a VW (also a bosch 3 wire heated O2 sensor), whats the difference bewteen this code and the 05-10 code ive gotten in the past when the sensors wernet hooked up properly?