Q: OBD ll, Oxygen Sensor Code on 1999 GMC Sonoma

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I keep getting a code for the rear oxygen sensor heater malfunction, the oxygen sensor has been replaced (2 times) The oxygen sensor is working fine. Also the SEC AIR SYSTEM is always incomplete and never OK. The oxygen sensor is incomplete untill it shoots the code twice then it shows as OK. If the SEC AIR SYSTEM causing the code to set ?
(1) Answer
This may sound dumb but make sure you are changing the correct O2 sensor. Second, make sure the power and ground to the rear O2 sensor is OK. If there are no secondary air system codes then that should not be the cause for your o2 heater code. Sometimes when fault codes are set the monitors will not run correctly. It is possible that the O2 sensor codes is affecting the monitors.