Hey guys. New to the forums. I have a little issue happening with my truck.
I have an 03 f150 with a 4.2 engine. Been getting the p0420 code and randomly my engine will bog down like the old carburetorated engines would.
I changed out the 02 sensor, thinking that was it. Nope. Took it to a local shop trying to avoid the dreaded converter replacement. The mechanic said the converter needed to be changed. It's a simple bolt up deal, so I did it myself. The truck IS running way better but I still randomly get the code and a check engine light. My code reader still shows downstream O2 sensor fluctuation, as it was before I changed the converter. My right side downstream is steady.
Any suggestions or advise?
Thanks so much and sorry if this topic is covered elsewhere.

 

P0420 Catalyst System Efficiency Below Threshold

Read more at: https://www.obd-codes.com/p0420

If the downstream O2 sensor (catalyst monitor) is switching like the upstream, the cat is chemically inert (usually) see the link above for more info.
.

 

Thank you for the response. It's a brand new converter. It's only been installed a week. Any other reason why the downstream might behave this way?

 

None that I'm aware of. It's not unusual for a new part to be defective. Ford ships vehicles off the line with brand new defective parts, what I was told is it's cheaper for them to pay the dealer to repair later than stop the production line to do it.
.