P0128 and P0420 on 2001 Volvo S40

[jesmed1]

Whatever you do, do NOT let someone talk you into replacing the cat before checking other possible culprits for the P0420 code. That code can be produced by a bad downstream (not upstream) O2 sensor that misreads O2 downstream of the cat and therefore mistakenly reports low cat efficiency. Have that sensor pulled and tested by someone who knows how.