I have been testing a Bosch automotive exhaust gas oxygen sensor of the zirconium dioxide type, which I suspect is faulty. There are four connections to it: two wires to a floating heater element, which require12v of no particular polarity; one sensor output wire and one sensor earthing connection to the body of the device.
The heater correctly takes about 1.7A when cold, falling below 1A as it warms up. The sensor is supposed to give readings between +0.2V and+0.8v when exposed to exhaust gases, the voltage becoming more positive with decreasing oxygen level. Instead, it gives readings between -0.8v and -0.2v, with the voltage becoming more positive with decreasing oxygen levels. In other words, the readings are behaving correctly but are displaced about 1.0v negative.
According to various websites, this is typical behaviour for a sensor which has become contaminated with silicone residue, and the obvious solution is to replace it. However, I can find no source for such a contamination and some websites imply that the contamination might be on the reference surface of the zirconium oxide 'thimble', not on the side exposed to the exhaust gasses.
My question is: does anyone know which side of the sensor is the contaminated one when the voltage is displaced negative?
I realise this is not exactly a 'design' problem, but I think that I shall only get a proper answer, as opposed to hearsay and guesswork, by asking designers who have actually worked with such devices.[I have already eliminated bad earthing contact between the exhaust system and the engine, which is the commonest cause of a spurious voltage readings. The vehicle is not consuming large quantities of coolant, so anti-freeze contamination is not very likely. To the best of my knowledge no repairs have been done with silicone sealants]