I drive a 2018 F150 XLT Supercrew with 3.5L Ecoboost. I have about 215,000 miles on it.

About a month ago I got a check engine light and code P0420. The light cleared on its own about a day later. About a month later it came in again and I took the following readings:


I cleared the code and decided to troubleshoot some more over the weekend when I had a temperature gun which was a few days later. On the weekend I measured catalytic converter inlet and outlet temperatures and got the following:

Driver side

Upstream - 473

Downstream - 621

Passenger Side

Upstream -458

Downstream - 612

I also got the oxygen sensor readings


Any thoughts on what is going on? It seems like I have some kind of intermittent issue. The first set of oxygen sensor readings look terrible but now they seem fine and temperatures seem ok too. Can catalytic converters fail intermittently or is this a sensor issue? Any help is appreciated.

 

Welcome to the forum.

 

it needs a cat, end of story, thats it, nothing else anyone says other than a cat will be the correct diagnosis...
passenger side....

if you can find a pid, cat 02 saturation capacity that will tell you life expectancy of other side...

 

I’ve had similar issues with my 2019 2.7L. Drove me nuts for the past several years. What I have learned through trial and error is that topping off the gas tank can contaminate the cats. No idea why but when I stopped topping off the tank my check engine light stayed off. After several months I mistakenly topped off the tank and the code came back.
This might not be your issue but it’s simple to check.
Good luck.

 

This makes a lot of sense. When the code has come in it is usually shortly after filling up with gas. Thanks for the info!