New to the forum (and Audi's for that matter). Hoping for some guidance.

'01 B5 S4 79k all original "time-warp machine", right down to the factory installed spark plugs and timing belt.

Just bought the car a month ago and it came with an intermittent P1113 code that set off the CEL but would clear on it's own every 300 miles or so. I just replaced both upstream O2 sensors with APWI brand units from Rockauto, now I have both P1113 and P1131 codes and can't get the catalyst test to go ready. The car runs great. No hesitation or miss. No other codes. 20.3mpg. Both of the new O2 sensors are returning .1 to .7V at idle. I'm reading with an ELM327 and EasyOBDII. I don't have VAG-COM and prefer not to pay to get a read if I can avoid it.

Is there another ailment such as a vacuum leak that can cause the O2 codes? Are the APWI sensors junk or are they just doing a better job of detecting another problem?

Thanks in advance