Njsalazar

I was hoping someone could help me with my 2009 Audi A4 2.0T with 135k, no mods, totally stock
About 3 months ago code 420 popped up ( prior to this engine mounts were replaced)
The local mechanic stated that the cat needed to be replaced and that the rear O2 sensor depending on its condition would need to be replaced.
Cat was replaced.
Picked up the car and 3 days later code appeared. Same code
mechanic replaced the rear sensor with Bosch OEM, front sensor

Picked up car and 3 days later same code
Identified that flow meter was bad, replaced

Picked up car 3 days later same code
Identified that the fuel was below recommened flow, replaced the filter. Flow back to normal
Picked up car 3 days later engine light came up. then disappeared for 2 days , then engine light came up. Took it back and its the same code 420
Mechanic is not sure what is next. Stated the all his tests are coming back fine, flow,exhaust etc.

Anyone have an idea of what to do next? not only replace but diagnostic testing?

Could I need to flash the ecu?

frustrated.. I miss my audi

Jameszhang

Same thing happened to me last month. Check the PCV valve, the problem solved after I replaced my PCV valve, which has a hole on it.

Njsalazar

and how would I check it.? Run the engine, take off the oil cap see if there is air rushing in? or out?
( had a clogged PCV valve in my old Xterra, kept blowing intake valve covers..)