[s-cars] Cleaned G4 sensor, and Idle switch connector (was getting code 2121) and.....

[David Forgie]

Jim:  Are you clearing the codes by pulling the fuse in the red fuse holder above the ECU each time
you "improve" something? ;>):
http://12v.org/urs/UrS6AboveECUFusePanel_1.jpg

As you chase the codes, you might want to read more about the various devices by clicking on the
labels (hot links) in this diagram:
http://members.shaw.ca/speedtek/UrS%20Engine%20Bay%20Device%20Map.html

Didn't you recently disconnect the ECU from the T55 connector?  Any chance that you didn't completely re-latch the connector? 
http://12v.org/urs/AANECUwithT55ReadyToLatchToECU.jpg

Here is an ECU pin-out with photos and links the various devices:
http://forums.quattroworld.com/s4s6/msgs/21009.phtml

Good luck. ;>)

Dave F.