It depends on if it's a 1.9 or 2.1  Ron (from BD) wrote and
said the bypass pipe for the 2.1 (replacing the cat) is NFG
because it doesn't have a fitting for the O2 sensor.
Therefore, the O2 sensor is in the cat.

To quote Ron:
"Putting aside inspection or environmental issues (and a
reminder that such a modification is not street-legal in the
U.S.), you should be aware that the bypass pipe does not
include a port for the 02 sensor.  This is not a problem on
1.9L Vanagons (as the 02 sensor goes in a different pipe
anyway, which would not be bypassed), but on 2.1L's it means
you'd have to tap/drill a hole for the sensor in the pipe
(or in another pipe in the exhaust system)."

Keith

David Beierl wrote:
>
> At 09:47 AM 1/23/2001, Keith Adams wrote:
> >It affects the O2 sensor, which affects your mixture.  The
> >O2 sensor is (as was mentioned) in the cat on the 2.1
>
> I don't think this is true.  The lambda sensor is ahead of the cat itself,
> even though it's in the same housing.  Putting it after the cat would
> confuse the poor thing terribly...
>
> david