Purchased a used 02 A6 a few years ago and after 2 years it began to self distruct. First thing to happen was the torque converter error for lock-up. Then it developed a ticking noise on the left bank which finally became a total engine destroying lifter wipeout.
I had paid enough for the car and the body and interior were in decent shape so I decided to try to fix the engine. Warning: this repair is not for the faint of heart. I ended up replacing the engine with a used unit with 89K miles and pulled the engine, transmission, front exhaust and subframe as a unit. The donor engine had some minor issues that were fixed before replacing. I purchased a rebuilt torque converter and replace as well.
At this point I decided that the total cost invested made it sensible to do it all so to speak, so out came the rear subframe for a refurbish. Long story short, the car is back together, everything works and I have no annoying check engine lights. Love the car now, but would not tackle this repair again.