I have a misfire 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
Re: O2 Sensor wiring
Posted: Sun Jan 07, 2024 3:20 pm
by soother
Hi Able. For that fault I would expect to see a VCDS code, maybe PO135 or P0141 sensor heater circuit bank 1 sensor 1, which is the one screwed into the exhaust manifold, or 2 (if fitted) after the cat.
I think the heater supply is under ECU control and switched through a relay, maybe by connecting the negative side down to chassis. There should be a steady 12V between the heater wires . I'm not sure if the heater is on permanently, or whether it is switched off once the engine is hot. I don't think the ECU is going to send ground (enable the heater) if the engine is not running.