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Thread: Oxygen Sensor Heater Circuit Voltages?

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    Regular Member marcdale's Avatar
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    Default Oxygen Sensor Heater Circuit Voltages?

    Hello everyone,

    I need a favour. I've been chasing a Oxygen Sensor Heater Circuit failure for some time now. Ended up having to fix a leak in the intake before I could progress but ultimately all 4 of the sensors end up giving a warning and eventually if I don't clear the MIL regular enough I get a generic heater circuit warning too.

    So, here is what I need from you.

    I know that on the Oxygen sensor there are 4 wires, I know that the 2 white are the heater circuit. Grey is the earth and the black on is the signal for O2 sensor. This is not a problem with with the grey or black because once the car is up to temp the O2 readings are fine and the heater circuit it only need while the engine is warming up.

    I need some one to check the voltages of each of the two white wires on one of their sensors please. Just one will do, to confirm my findings. BEFORE you start the engine but with the ignition on you should get 12V across both white wires. But when mine gets up to temp 1 of the white wires stays at 12V and the other one drop substantially and bounces around eratically from 2v to 9v ish.

    Can someone tell me if their 3.2 petrol does the same?

    12V through both white wires before before you start the car
    and then different voltages across them when it's up to temp.

    I'm thinking there is a control until or a relay some where which can't maintain 12V to both circuits, or something along those lines anyway.

    Thanks for any help you can give, this is doing my head in!

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    Regular Member marcdale's Avatar
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    I'm reading else where that the voltage passing through the heater circuit on an O2 sensor will reduce as it gets hotter because of the material used by design that increases resistance the hotter it gets. So basically I think all is ok with the heater circuit. I'm also reading else where that the heater circuit is switched off when it's told partly by the coolant temperture sender that the engine is warm enough, it then switches the heater circuit off and by then you should be getting a reading of the O2 sensor because it's now hot enough. So I'm looking in to the thermostat and coolant sender as they may be switching the heater circuit off too soon or too late. It's done 120k now, so I guess stuff like thermostat and temp senders are bound to start playing up at some point.

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