About two months ago, I purchased a 2003 Audi A6 2.7T Quattro with 160,000 from a dealership against my better judgement. When I test drove it over the weekend I really enjoyed the car. The ride was smooth, transmission shifted well, but the acceleration was really underwhelming for a bi-turbo. FF two weeks, check engine light came on and with my dads cheap diagnostic tool I got a reading for P0420: Catalyst System Efficiency Below Threshold (Bank 1) (Going from memory on that, not 100% if that was actually it.) After a quick google search, most likely it was the cats. Cleared the code and then a week later it came back on for the same error and additionally for the MAF. We replaced that and it didn't help at all.

While we were replacing the MAF, I noticed that my coolant levels were WAY below minimum. Maybe half an inch in the tank. It also was filled with the standard green coolant. We took it to a shop were they found that the heater core was bad (which I read somewhere that it is common for these to go bad with the wrong coolant). They replaced the coolant, the heater core and then gave us back the following error codes after using their diagnostic tools:

"-p1453 - EGT Sensor 1
-p1557 - Charge pressure control
-p0103 - MAF
-p1143 - Load calculation
-p1458 - EGT Sensor 2
Further diagnostics is required"

I know that I need to sell this thing, so I took it back to the dealer where after talking with the salesman we agreed that this vehicle shouldn't have been placed for sale in it's condition. They have agreed to work with me a little, and they are willing to take it back for a few hundred less than what we paid for it. However, I would like to sell this for private party so that I can get the most cash back for my next vehicle, that is if I can get it into proper running condition.

Does anyone have any idea what could be wrong with all of this? I did a little reading on these forums and I think that the car might be in "limp" mode because of the EGT sensors, but would this cause all of these other errors?