Check Engine Light Keeps Coming On After Dealershi

I have a 2001 audi a6 4.2, same problem with engine light i was given every thing underthe sun to try, like better gas, not fill the tank to the top,not letting the engine idle,replace a ir filter more often.replace o2 censor, and yes when the warranty was over by 600km's 375miles it was remarkable how the audi tecks then found that the head gastket was leaking under pressure, the coolant mixed with the exhaust and messed up the 02 censor, and your right you guessed it I paid the $5,500.00 to get it fixed and it's fine now, hope you have better luck than I did. need less to say I'm not happy with Audi and I'm trading mine for an American made car.