Oil on the spark plug - could that be making a P0420 worse?

2006 TRD OR 4.0 Automatic - very minimal rust - I bought in California, wrecked it and rebuilt it myself

So I've been battling my P0420 again. At best I've gotten it to go away for a couple months but now it seems to be coming back every other week.

My latest observation is.... it happens after the truck is warmed up and I park. After it cools down a bit and I restart it, the truck runs a little rough. It'll be the smooth for 2 seconds and 1 second of vibration then smooth again for 2 seconds and so on. When that happens my P0420 lights up the check engine light.

I just changed my spark plugs but no change really. I did notice I had a decent amount of oil on the 6th cylinder plug. Not too bad but it was def wet. I can't post pictures cause I'm using a computer at work. I'm back to living in my truck. That said I'm saving some money to get the valve cover gasket fixed. I figure while I'm at it I'd get the timing chain cover leak fixed. It's behind the power steering pump. it's not leaving a drip but it's def mucking things up with oil on the engine and rock guard below.

If I'm gonna get all that done I'm kinda figuring to replace the Head Gaskets, Valve gaskets and whatever other gaskets I can. Maybe a new timing chain and guides. I got the full gasket kit from Toyota hoping it'd help me learn more about my engine. But aghh I'm not feeling confident to do the work myself without a place to do it. I'm in the Chicago area and that guy on YouTube "The car care nut" is about an hour away. I think he'd be expensive but do a quality job.

Do you guys think that it'll help with the P0420? I'm hoping that will fix the rough running at least....Let me know what you guys think. Ring wash? The truck has a bunch of small issues but nothing too major. Even with the check engine light on it does good in between the times it runs a little rough. I'm thinking I should address it before something fails. What would you guys do? Thanks

 

thats interesting, i recently did a plug change on my '13 and had a good amount of oil on #6 too. Rest looked great. Wasnt throwing any codes at me either

 

P0420 is a Catalytic convert issue. If it was a bad spark plug, you'd more than likely get a mis-fire in Cyl #6.

When I got that code in my first gen, replacing the o2 was a weird temp fix but the DTC always came back. Changing the catalytic was the correct long term fix.

 

Like said the Po420 is often a bad cat converter. It can often be set under strenuous conditions that push a decent/good, but not 100% functioning (worn/used up) cat converter. Toyota seems pretty common dealing with this. IMO An over aggressive epa standard related to oxygen saturation more often than not set the po420. An aggressive/hilly/high speed drive can over ride the epa set o2 saturation capabilities.

Will huge money on a brand new overpriced oem converter fix it? Yes/depends/Not always. A cheap universal? Some times/Depends. Can cheap o2 spacers? Often/Depends.

My money is on the cheap o2 fix first. Even if it lessens the po420 occurance its worth a try,

 

I'm not familiar with numbers but I had oil on spark plug on the middle cylinder on passenger side recently. The valve cover bolt turned out to be loose. Hoping that's it. No codes either.

 

Sounds like you have leaking valve cover gaskets, leaking oil onto the plug.

P0420 is cat inefficiency. Caused by things like an air leak or worn out cat.
The first easiest thing to check (and how vehicles are repaired) that is also required in general, is verifying there is no exhaust air leak.

because whether or not you have a bad cat, if you have an issue causing improper AFR, it will not matter if the cat is good. And cause such codes. Such as/including but not limited to things that can cause such codes like an air leak. Bad O2 sensors. That get old and need replaced regardless. Etc.

I had such a code once. I found and repaired an air leak. The code went away. Also replaced the O2 sensors because they are old, which is reason to replace them.
All much easier and significantly cheaper than dealing with a cat.

if I’m going to condemn/diagnose a cat is being bad, other things are gonna be tested and ruled out first.

Cats don’t last forever.
Burning oil accelerates wearing them out.
Supposedly one test is measuring before and after temperature at the cat. Though I’ve never heard of this in the professional world. I’ve heard of ruling everything else out first.

 

rebuilding the engine is not a cheap endeavor, timing chain/head gaskets/ timing cover etc, unless its burning coolant (sticky sweet smell on start up usually first sign) save you money, for the timing cover fix just remove pump (dont unhook lines) push it aside, wash down the seam of the timing cover, lay a bead of the permatex "right stuff", it will not leak when this has been done, I do have a post here when I did my head gaskets I showed a machined defect in the timing cover near the PS pump. 1hr or less vs 6plus to pull the timing cover no brainer