Funny you should ask, I had the rare urge last night to spruce up the oxy-sensor file with some of Tim's comments... This is what I added:
Original Archive Name: oxysensor
ND BBS Archive Name: OXYSENS.TXT
Here are a collection of notes on the oxygen sensor, what it does, how to replace it, how to diagnose some related problems, how to turn of the red light that comes on after 30/60k miles.
Briefly, your oxy sensor (aka Lambda sensor) sniffs your exhausts for oxygen content, and feeds back this value in the form of a voltage to the car's ECU. The ECU in turn adjusts the fuel delivery to meet the desired fuel to air mixture value. This is also why this is called a closed loop or feedback system. Just like in the BMW commercials, VWs tune themselves, and have been for quite some time...
Oxysensors wear out, and if they do, it usually result in a rough idle,
poor engine performance and higher fuel consumptions.
Besides these annoying side effects they may also cause your
catalytic converter to burn up or loosen up.
A broken up catalytic converter sounds like a couple
of coins rattling in a coffee can. I call it the death rattle.
You do not want this to happen, because the cat conv., depending on
the car, may cost up to US$1000 to replace, and in many states
like CA it is illegal to buy a used one.
Some just gut it with a crow bar (the ceramic honey comb is surprisingly
strong), but that's very illegal (US$20 000 fine if caught) and will result
in more pollution.
Newer cars actually expect the slight cat back pressure and
therefore end up running *worse* with the cat gutted.
To remind users that the oxysensor may be on its final snifs, VW installed a little box that contains an odometer that trips a little switch at either 30 or 60k miles. The older cars with non-heated sensors have to be changed around 30k, the newer cars use a heated sensor at a different location thereby lengthening their useful life to 60k miles.
Many just reset that odometer to save money w/o replacing the sensor, and instead burn up the cat costing 10x more. There are ways to check whether the oxy sensor is still good (it depends on your driving, they may last much longer, see Bentley and below), but if you want to be sure, replace it when needed.
VW uses different styles of oxy-sensors, from the 1 wire (cheap), 3 wire (2 are used to heat the sensor at start) up to the 5 wires used in the Corrados. Naturally, each wire adds about 40US$ to the cost, and some claim you can use a 1 or 3 wire sensor instead of a 3 or 5 wire respectively if you do not care for cold running behavior. I would not advice it, but if you drive a beater and have no cash...
It's a good idea to use a bit of anti-seize on the oxy-sensor threads before installation, but don't let any of it touch the sensor because the paste contains lead. Similarly, keep products containing Si away from the sensor. It's a very sensitive piece of equipment.
Jan Vandenbrande
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