take a look at what the MAF values are.
did you clear the code after the new maf was installed?
the cat code can be "fixed" one of 3 ways.
1. put the cat back on
2. get a 02 sim (like Aaron said)
3. buy a PT and remove the code from the pcm
1996 with 254k miles, L32 4" FWI -> ported N* -> Ported Gen V w/3.0" Pulley, Stage 3 Phenolic I/C, ZZP FMHE, 1.84 RR, Headers and 3" pipe to mufflers, F-body brakes, and lowered on Eibachs. -RIP
AMG C400 White on black. Stage 2 w/E30 - 11.9@117 -daily