My 2002 A4T has had a on going service lite issue. Code PO420.
I have replaced both oxy sensors with oem bosch. Code returns within 15 miles.
I then replaced the Cat converter with Oem new. Code returned again . Replaced the trailing oxy sensor. Code is back..? Any one faced same issue..? Car has 104,000 miles. Not an oil user. But it did incurr the oil sludge problem.
Thankyou
Cory

 

Try Seafoam in the gas tank, and through a vacuum line.