I bought a1996 z last year. A4. My ses light keeps coming on w/ a codep0420 and p0421. What is catalyst sys. efficiency? How do I fix it. Have new plugs, put a new bank 1 O2 sensor in and a new fuel filter. Also, what kinda #'s should I see for performance and economy?

 

Either your drivers side catalytic convertor is deceased, or the after-cat O2 sensor on that side it faulty. With the 2 related codes, I'd bet on the cat.

 

When under my car, it looks as if it only has one cat, unless its the funky looking can thing right below the driver o2. My exhaust is manifolds, pass side to conv, left side by-passes conv and goes into a single inlet flowmaster. I think the kid who owned it before me put the wrong muffler on it, single inlet/outlet.