i can only guess - but the coolant temperature sensor failed very often on those models
since the audi is [http://en.wikipedia.org/wiki/CAN-bus|CAN-bus] controlled, i would use the A/C or my CAN-bus reader to get the actual error codes. i didn't found a good source for the A/C trick in english - only german sources - here is a google translated page: [http://translate.google.de/translate?u=http%3A%2F%2Fwww.a4-freunde.de%2Ffaq%2Ffaq_klimacodes.shtml&sl=de&tl=en&hl=&ie=UTF-8|CLICK]