Permanent P0420

Alexwallace23

P0420 is Catalyst System Efficiency Below Threshold (bank 1). Just by looking at the code description you would think it's a cat converter problem but the possible problems are:

Faulty catalytic converter
Engine misfire
Fuel injector leaking
Engine mechanical damage
Oxygen sensor that has improper readings
Exhaust leak

I think it's an oxygen senor issue since the code comes and goes after a day. I do have HFCs with a tune. This code first came up after about 12,000 miles after the HFC install. I actually went a couple months after the HFC install before I got the tune and I didn't have any CELs.

I will test the O2 sensors soon and see how that goes.