Intermittent Check Engine Light

[rottenma@ei.dupont.com (Mike Rottenborn)]

The Check Engine light in my '89 200 Turbo comes on intermittently,
especially when the car is either cold or is under heavy throttle.
Has anyone else seen this?  I have owned other cars that do this when
the oxygen sensor needs replaced, but on the Audi I wouldn't be surprised
if the equation was a little more complex, possibly involving the knock
sensor or other components.

If the oxygen sensor is the culprit, has anyone replaced theirs?  Will the
22 mm socket with the slot for the wiring pigtail that many auto parts
stores sell (for $13 - ouch) do the trick?  Can you recommend a good
source for the sensor, aside from paying $150 at the dealer?

The flood of questions continues.....  If it's not necessarily the O2
sensor, I would guess that driving until the light comes on and then
activating the fault memory of the computer would provide some clues.
The Bentley manual talks about bridging some terminals on the top of
the fuel pump relay to activate the codes, but it calls for a special
VW test jig to display the codes.  Can this be done with a simple 12V
LED?  Is this a fairly straightforward process?

Sorry about cramming all these questions into a single note, but I have so
many that I like to save them up for a while rather than posting every
day.  Thanks in advance for any information you can provide.

Take care,
Mike Rottenborn

P.S. The car has 73K miles and I believe the O2 sensor is original.