Engine Cel

Cel

Didn't know where to put this, but i have a CEL that reads Catalytic Effeciency below threshold. Would you guys know what would cause this?
I always get the code deleted, but comes back when i really baby the truck, I have it Supercharged and the only fuel mod i have is an fmu.

Code P0420 - Catalyst System Efficiency Below Threshold

Possible causes:
- O2 sensor is none functional or lazy
- ECT (engine coolant temperature) sensor is none functional or lazy
- Leak in exhaust manifold, exhaust pipes
- Screwed up timing (too far retarded)
- Bad catalytic converter

Generally this code pops up now and then, but causes no serious drivability issues. If you live in an area that smogs the vehicle - this will have to be fixed.

Thanks
I've replaced the rear O2 cuz it went out, i'll check the rest. but it just bugs me that the lights keeps coming back on

usually what you should have done before replacing the 02 sensor was decarbonize your engine usually carbon deposits would sit on the 02 sensor and make your sensor act "lazy". but replacing it works too.

I replaced the rear O2 b/c i tried cleaning it before replacing but it didn't work. Yes the O2 was lazy it wasn't switching fast enough. But i still have the one code come back. I'm gonna check the front one, but the weather is crap when i don't work.
I don't have smog checks but the light irratates me