I'm seeing code P0161 sensor heater circuit failure. LH rear sensor.
Resistance for heater circuit is around 9 Ohms.
( AudiWorld Tech Articles)
Voltage from ECM reads 3v, with the engine off and ignition on. These readings are identical to what I see on the RH sensor where there is no error code.
So now I'm a little stumped.

I should mention that I have done the diode modification because of earlier P0421 and P0431 codes. per here:
AudiWorld Tech Articles
I then got 'no activity' codes on both sensors. This looked like I had the diodes reversed (but I didn't). Nevertheless, I reversed them both to see what would happen and the 'no activity' codes cleared BUT left me with this single P0161 left rear heater sensor problem.
Car runs great otherwise. Any ideas anyone?

 

Just one other piece of the puzzle.
I had always read that the ECM sets P0161 whenever it sees zero current or too high a current draw on the heater circuit. i.e. it then knows the heater element is bad. However, this site OBD-II Trouble Code: P0141 Oxygen O2 Sensor Heater Circuit Malfunction (Bank1, Sensor2) suggests that the ECM is using reaction time to decide whether its OK.
i.e If say, it takes 4 minutes before the rear sensor starts reacting to fuel changes instead of the specified 2 minutes (or whatever the heater makes it) then it flags the code. Of course, in this case it would need a way of distinguishing the P0161 from the, sensor never working at all (P0431) code. I guess just waiting longer would reveal that.