Audi Repair: Malfunction Indicater Light On, audi a6, gas cap

Question
My car is a 2000 Audi A6 2.7t. The first time the MIL on, I went to the audi dealer for the code. It was P0442 and an aftermarket gas cap was found. Then I replaced it with a brand new gas cap, but the MIL have not gone off after several day's driving. Then I went to the dealer again, and they run the code again. This time it was P0422 and P0455. They cleard the system and the light went off. (I did not see any repair work on the problem, maybe just reset the code). After one day's drive, now the MIL is back on. I went to the dealer and they said they have to do diagnose again. But I think it should be the same problem which they have not fixed the last time. What should do with it? Let them do the diagnose again? However, every time I do diagnose there, it will cost me 60 or 70 dollars.

Answer
hello
For me it sound like they do not know what to do.
I do not know what is P0422 I think you wrote it wrong, but P0442 and P0455 are for EVAP system. EVAP system is part of fuel system and it makes sure there is not any leaks (evaporations and liquid). This system can be tested using Audi scan tool, so technician will know if the system is fixed or not. You have the same problem coming back, I will talk to service manager and see what he thinks. They may say that MIL light may come ON by 100 problems, but You tell them it's still the same problem and you know the system can be tested for leaks.
there are 2 tests they can run #70 and #71 , once they fix the car tell them to show you the print out with faults and these 2 tests . On both  it has to say SYSTEM OK.
good luck.         pltech