still get p0420?

oh ya. lights flicker from time to time. Did a couple of charging test and first on said no load @ 15.29v load V @15.18v charging ripple @ 80.0mV
sec test was no load 14.75v load V 14.75v charging ripple @28.0 mV could this tell the ECM to not pass the cat sys?

So can I ask if you have an exhaust leak before the catalytic converter? If so this will cause the issue.

https://2carpros.com/articles/why-an-automotive-engine-will-run-rich/

Also, I would remove the plug plugs to se if any are black, this will tell you there is a fuel injector that is hanging open on that cylinder. Please go over the guide above and get back to me. Also, if you can upload a short video of the engine running at idle it will help us.

I have done a smoke test found no leaks. Sparkplug in order left to right. Put them back the way they came out.the catalytic is a aftermarket davico 19518. Don't have the funds for a real catalytic. Which is close to 2k.but hopefully can get this fix for my son.

Good find on the spark plugs, The one looks dark, I would check the compression of that cylinder and if it is okay replace the fuel injector. Here is a video to help.

https://www.youtube.com/watch?v=KOxIm1F8AW0

Please let me know what you find, also the after market cat might be the issue too but lets do the test first.

Compression test from left to right 220 210 209 210 seem pretty good to me.

Fuel injector listen to all of them only one sounded a bit different the one plug u said was darker souned different so will replace and probably change plugs too

Drove about 25 miles and p0420 came back on. Probably is the aftermarket catalytic. I did ended up changing the plugs and replaced the one injector.

Yep, the compression is good. Does the engine have an exhaust leak? If not I would go with the aftermarket cat causing the issue. It is rare but I have seen the PCM go bad and cause this issue as well. Also, the engine seems to run okay, no power issues?

It runs great with no power loss.

Okay, if you have no exhaust leak I would say it is the aftermarket cat. Lets try that first.