Idle Switch

[Brice Warnick]

I had the same problem but I had code 2212 for the throttle and lost
boost. It turned out to be a broken wire from pin 2 in the harness to
the ECU.  My 2212 code vanished when the short to ground was repaired
and my boost came back.
With 2121 code for the idle part of the switch you probably still have
boost.  Right?
If you have a new switch then you could possibly have a bad new switch.
Your throttle switch could be out of adjustment. Loosen the two screws
and follow the procedure at 24-120-1 in the Bentley.
Your ISV could be sticking. Look inside it and see how dirty it is.
Your throttle cable could be out of adjstment.
You could have moisture in the harness. Use Stabilant 22A on the harness
and reconnect.
Or your floor mat could be pressing against the accelerator pedal.
Or you could have a short to ground or positive.
To verify you have to check the connections in the throttle harness. If
you find a bad one you the have to pull the ECU harness and check it
both ways to see if it is a broken wire betwwen the throttle and ECU
harness.
First try removing the ECU harness and putting it back on as it can just
be a bad connection and just removing the harness connector and putting
it back on will sometimes temporarily clean it.     
I am currently using a new S4/S6 throttle switch
as a new 200 20V switch was unavailable. 
the S4/S6 switch part no is 078 133 154 and the Bosch no. is 0280120431.
I picked one up for $50.00 new on Ebay and wanted to try it. It works
great. The boost comes on a little bit earlier. The switches look
identical.
The 200 20V part no. is 034 133 154F and the Bosch no. is 0280120410.
This switch just got ordered by Audi to Bosch on 8-28-2004. It had been
out of production by Bosch for a year or more. So the lead time on
getting more switches is months away. There are none of these switches
anywhere in the world now new.
What switch part no. are you using?