DTC P0161 - Left Rear Oxygen Sensor Heater Malfunction: Condition

WARNING: This page is about a different car, the 1997 Mazda Millenia. However, it is still accessible from the selected car via links, so may be relevant.

DTC is set when ECM voltage is greater than 11.5 volts when voltage is supplied to left rear oxygen sensor heater. When ECM voltage is less than 5.8 volts when no voltage is supplied to left rear oxygen sensor heater. Possible causes are:

Left rear heated oxygen sensor.
Open or short circuit between main relay and terminal "C" (Red/Black wire) on left rear oxygen sensor.
Open or short circuit between terminal No. 3G on Engine Control Module (ECM) and terminal "D" (Black/Blue wire) on left rear oxygen sensor.