I have a misfire on my 1.4 AUD 6n2 that I have tracked down to what I think is a problem with the supply voltage to the o2 sensor heater. The voltage varies bettween 3.8 and 12 volts, (IE, 3, then 10 then 4 then 12 ect over a few seconds) It should be 12 volts all the time? Does anyone have a wiring diagram that shows where the other end of the o2 sensor heater wires goes, I know one side goes to earth. does it go via a relay?
Cheers
Able

Edited 5 January, 20242 yr by able
typo

Do you have white/red and brown/red wires on pins 3 and 4 respectively of the upstream sensor?

If so, the white/red gets a 12V feed from fuse 1, and the brown/red is 'earthed' via the engine ECU (pin 1).

This is typically a Pulse Width Modulation arrangement which effectively means that the applied voltages vary under ECU control depending on the heating required at any given moment.

 

If you're meaning the downstream sensor, then the return to the ECU (pin 13)  is brown instead of brown/red, but otherwise same.

 

Apart from seeing a non-constant voltage, what makes you think the O2 sensor heater is involved in the misfire problem?

VCDS showed a misfire on 2,3,4 cylinders. I Gave the engine an oil change, oil filter and air filter. Changed the plugs and leads. But still had the misfire (2700-4000 rpm) From cold it is ok, and till around 8 minutes later when it will misfire (when warm/hot) so, changed the O2 sensor and ignition coil module and now a new ECU, but still has a misfire, changed over all vacuum hoses, did a compression test (all good 160-163 psi) but still the same consistent misfire at same rpm range. Decided to change the MAP sensor, but strangely with the MAP sensor disconnected it runs fine? So, new MAP installed , but back to the misfire? But with a disconnected MAP sensor it runs fine (cold/warm/hot) but I am getting an open circuit heater  error now with a new(tested)  o2 sensor?

OK, that's some good info to mull over. 

Will let you know in the next day or two if I have any ideas.

Probably worth checking the sensor is getting the voltages and earths it's supposed to have. I don't know in this case what those would be though. 

Its strange, a new  o2 sensor is giving the same error, so I put a 12v bulb across the heater wires to see if this was working (like Pete1 said “This is typically a Pulse Width Modulation arrangement which effectively means that the applied voltages vary under ECU control depending on the heating required at any given moment”) so, if it varies I could do this test with the bulb, and it flashed and with varying brightness, so I assume this is working ok. I tested the sensor (all wires - heater/probe) with a meter, and it’s all within limits? Anyway, I’m taking the new o2 sensor back to exchange for another now.

Are you sure the heater error isn't for the downstream sensor?

 

The error is on Oxygen sensor bank 1 sensor 1 (Before cat)

Fair enough.