Rep Power: 0

Guys, i installed a new aftermarket cat, just after driving roughly 5-8 miles i got a CEL, checking it with a scanner it is

P0420 - Catalyst System Efficiency Below Threshold (Bank 1)

Took it to the mechanic, he checked the O2 sensors reading and according to him the second o2 sensor needs replacing as its giving quite high voltage around 1.2V. Although the sensor does fluctuate a a lot meaning its not dead or anything
Car runs fine without anyproblem, only yjhe fuel efficiency has dropped last time i didnt get a reserve light even at 370 miles with a full tank , this time i got a reserce light at 340 miles only.

Does it really mean a new sensor, they are also not that cheap, just afraid that if i replace it and the CEL comes back again .

Any help appreciated

 
Rep Power: 0

old post, but same question????

new cat.... P0420.

i think i'm gonna do the 2nd O2 just cause i've got 160k on her...
so we'll see.