>     Until recently I have always heard that the O2 sensor doesn't make
any
>     difference.

You heard wrong.  The O2 sensor is a vital part of the fuel injection
system, and plays a large part in helping the ECU determine the air/fuel
mixture that the engine gets.

  Now my light is on and the van continues to drive fine.
>     Is it worth replacing it?
>
The light has nothing to do with the condition of the oxygen sensor.
The light is merely tripped by a mileage counter every so often.  The
oxygen sensor doesn't talk to the light and the light doesn't talk to
the oxygen sensor.

You can get under the front of your van and find the mileage counter box
in line with the speedometer cable.  There's a white button that you
push to reset the light.

Sean Bartnik
Fairfax, Virginia
============================
'81 Vanagon L Westfalia
'74 Karmann Ghia convertible

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