sbklf

Last year I had P0136 and P0420 codes, that did not go away with new O2 sensor, pointing towards catalytic converter issues (on the right side). I did the spark plug anti foul adapter trick on the O2 sensor and the codes cleared. Yesterday it started running rough and gave a cylinder misfire detected on #4 cylinder. Turned out to be build up on #4 spark plug, a new plug corrected the problem. Did not intend on running forever with the O2 sensor trick, just been putting off going any further with it.

-Is it likely the tricked O2 sensor caused the buildup on #4 plug?

-Is it likely the catalytic converter(s) are bad on that side?
If so, any advice on converter replacement would be appreciated (Can I replace one side only? Which after market converter would you use?...).

I took the adapters out this morning and expect to get the P0136 and 420 codes back. That #4 plug is hard to change, I hope I haven't caused a bunch of buildup in the cylinders on that side that will cause more fouled plugs.

#4 plug less the buildup (knocked it off accidentally)

ProjectSHO89

No.
Yes. You learned that previously.
Wouldn't bother. Just continue running the anti-fouler. It's probably not your current problem, see below.

See how rusty the top of the plug is? That suggests that water or coolant is getting down into the plug well and is probably degrading the COP boot and causing the misfire. There is a coolant elbow/hose joint immediately above the #4 cylinder that, if it leaks, will cause your exact issue.

If the misfire continues, the converter will probably (eventually) get so much excess fuel in it that the substrate either melts or disintegrates and then you WILL have to address the issue as the exhaust will become plugged. Tap on the converter with a rubber hammer and listen carefully. If you hear the sounds of crap moving around inside the converter, it's already too late.

sbklf

That I know, the only issue the engine has is that it seems to run rich (noticed at idle) and I thought it was possibly due to a false O2 sensor reading and that changing the converter would correct it. Sounds like it might be the other way around where an engine issue caused the converter failure. A little history, I got this truck out of a barn where it had sat for 7 years. Engine wise I basically replaced the tank and fuel pump to get it running. I did everything on the cheap since I do not know its history but have more confidence in it now. I haven't even changed the plugs but will tonight and plan to remove the injectors and clean them (if that is possible).

-Will the condition of the plugs tell me if I have other issues that might ruin the converter?
-Can a misfire be so slight it can go unnoticed?
-Factory is two converters on each side, four total. I would try to replace the converter on the one side only. Which after market converter should I use if I go to a single?

DarrinT04

I dont think changing the plugs will do much in this case

sbklf

Changed the plugs and took out the anti-foulers. Runs a little better but still smells rich.

ProjectSHO89

A converter that isn't working will leave unburned fuel in the exhaust.

Get your scan tool out and check your fuel trims. You have to be able to see what the computer is seeing, your nose won't tell you enough.

Without the anti-foulers, the P0420 will likely be back soon.