Diagnosing O2 code - what are my voltages telling me?
 

Hi,

My 2.0 N/A GH7 (150) 2009 occasionally throws up a code for P1038 - it's high voltage for the rear 02 sensor.

Before just buying a new one (bloody he'll, the prices of a branded one now!!), I thought I'd just check out the wiring incase that's the culprit.


Testing wires back to ECU from the socket next to the air box.

​​​​​​This is what I'm seeing, engine off, ignition on. Engine/coolant cold.

Sensor 1 (not throwing a code): +10.5V, -10.5V, 0V and 0.5V.

Sensor 2 (occasional P1038 code, no other symptoms): +8.5V, -8.5V, 0v and 0v.


Sensor 2 has been recently replaced by the previous owner, I suspect to solve this issue...and hasn't worked. I suspect it's a cheap sensor which could be a problem in itself, but I think the lack of a 0.5v on the rear sensor is telling me no reference signal from the ECU and therefore I have a wiring issue... right?

Really grateful for any thoughts.

Cheers

What you really need to do is look at voltage with engine running, and checking voltage change when you rev it (full throttle "blip" ):

The signal voltage should increase as the engine accelerates, then decrease on the overrun.

The pre cat sensor will respond quicker and give a wider swing of voltage as it switches from rich to lean whereas the post cat sensor will more or less follow suit but at a lesser extreme and a bit slower...depending on the cat temperature.

Not sure how you are obtaining negative voltages...you should be checking between one wire and the chassis.

Two wires will be heater (supply and ground, usually both the same colour), One signal, One signal ground.

Personally I'd check OBD live data first, as that's what the ECU is "seeing". If it's pegged at a high voltage then you can confirm it with the multimeter.

Code is actually P0138

See attached pdfs