'03 Impala, 3.8 66k miles, P0420 code
Started out as a plugged cat issue, cat was replaced by his mech.
.(brother's car and I don't wrestle with exhaust work on jack stands)
Low cat efficiency bank 1 code popped up a few days later. Changed front O2
sensor, fine for a few days and same code came back. Changed rear sensor
thinking it may be off enough to trigger a false low efficiency code but not
a fail code and possibly got damaged when it was removed during the cat
Fine for almost a week and now the SES is back on again. Haven't had a
chance to read the car yet but I'm betting on the P0420 being back again. No
other codes were set with the P0420.
Pro's wadaya think? New cat bad?