My 2015 F150 has about 185k miles. I have had a very intermittent P0420 code for like a year. It would come on for a little bit, then turn itself off. I recently had the 4WD system serviced and also had a coil and plug replaced on the #5 cylinder due to a reported misfire. I told them about the P0420 but they said there was no code in the history so did not try to diagnose it. I had an O2 sensor replaced last year but I can't remember which one.

Now I have the P0420 again. I have tried a can of cat clean and running higher octane fuel as well as going on a couple long drives to make sure any excess fuel/soot that may have been happening due to the misfire are cleaned up. The code has gone off briefly but now seems back permanently.

I am not sure how much longer I will keep the truck, probably a year or less, so replacing the cats seems like an expensive repair that probably won't be recouped unless I just drive the truck as long as I can stand it.

Are there any other cheap/easy fixes worth attempting? How can I diagnose if it is for sure the cats and not continued misfires or a sensor issue? I've done coils and plugs before and the rear cylinders are a pain but I think I'd be willing to do it again if it would get rid of the code for the long term. The. Again, at 185k I could imagine maybe my cat is just spent and there's no getting around it.

What say you auto board?

To add some context: I was lazy and just trusted the shop to diagnose the 4WD issue. I was getting a grinding noise under load and they diagnosed it as the IWE valves and replaced them and the solenoids (I think? Going by memory), as well as the coil and plug and a couple other small things. All in it was about a $3500 repair bill, so dropping another $1500-2k on cats for a truck that is only worth $10-12k (miles and accident history) is a hard pill to swallow.

You can use a temperature gun and check the temp of the cats and see if it is functioning. Lots of YouTube videos explaining it. Find a local mufffler shop and they can put a universal cat on it for $400 or less.

P0420 the majority of the time indicates the cat is actually bad.

Only two ways for sure to get the light off - replace the failed cat or get a tune that disables the P0420 code. Neither are really inexpensive.

The cheap option is to modify spark plug non-foulers to space the sensors out from the exhaust stream. Works most of the time.

https://www.f150forum.com/f72/how-trick-pesky-o2-sensor-47908/

If the cats are really clogged, you might run into performance issues with the truck doing this.

EnviroAg96 said:

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You can use a temperature gun and check the temp of the cats and see if it is functioning. Lots of YouTube videos explaining it. Find a local mufffler shop and they can put a universal cat on it for $400 or less.

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I have an IR temp gun, so this seems like an option. If I can get new cats for $400 that seems like a no brainer.

I found putting everything I knew about my codes in to grok would give me very good troubleshooting and source tips.