Did a rear passenger O2 sensor ( 2000 quattro with 2.8) which solved several obvious Check engine codes. Was all clear until the occasional p0431 code came up which had not been evident previously. Cleared it with the reader & it was Ok for a week. Repeated same which would last a day or week, once including P0421. Not much internet solutions on these two codes except "replacing cats". As I just had an excellent e-test and light would stay out, figured it was time to fiddle. Long story short, found secondary air injection hose clip at bottom of TEE at rear of engine loose. Clip would not secure & it was likely rattling loose occationally. Wire tied the SOB up tight & all is well. Hope this pops up on "searches" & others benefit from my folly. Yes I had that clip apart to nicely route the O2 sensor installed recently but thought it was secure when reconnected. In short, suggest checking these secondary air injection lines/clips before jumping into CATs. when getting occasional P0431 or P0421 codes.
By the way, can anyone tell me the proper "trade" name for these plasic hose built in clips/connectors.
They are a pox on mankind. Never do they want to release and usually a bear to get back on. The engineer that designed ( designed??) these should have his head caved in flush with his shoulders using a 1 1/2 " wrench. (yes imperial, just seems appropriate. )
Yes I also get the fact that they are more for quick assembly during construction but that does not make it right.
Cheers
TheCounty