P0420...

[Flying Hamster]

Melted a plug wire - 2006 4.3 130K miles
loom came loose and wire cooked on the exhaust manifold.
replaced the wires, plugs and now I get P0420 - generic code for catalytic converter on bank 1 not meeting minimum standards.
Is it related? Will the fouled convertor burn clean or am I looking at new converter?
Should I go outside and throw rocks at the truck?

[GM-power]

You need a cat.

[Deer_hunter_088]

yup killed the cat. my s10 did this. theres ways to get past this without a new cat. but im sure you know em.

[kwmech]

Check the voltages of both the 02 sensors before you go throwing a few hundred dollars on a convertor. Maybe you knocked a small vac hose loose while changing the wires which would throw the voltages off

[95 S_Trucker]

You would get a P0171 and/or P0174 if there was a vacuum leak.

Lean codes will set before a catalyst efficiency code.

99% of the time, a P0420/P0430 is caused by a converter issue.

I think its converter time, especially since the OP had an issue that allowed unburned fuel to enter the converter.

[Flying Hamster]

Well, the truck started sputtering above 3000 rpm, and then backfired loudly several times and resumed running normally. Cleared the light, it came back, and now at idle it sounds like someone rattling chains in a metal trash can. With the truck off, I can get a similar rattle from the convertor by smacking the exhaust...
Yeah, cat is dead.
Still thinking about the rocks.

[Pyro]

If you melted a plug wire then that cylinder was likely not firing, which means it's unburned fuel would be going straight into the exhaust. The P0420 code would be normal for that situation, but I'm surprised you didn't get a misfire code as well. I'd clear the code and see if it comes back after putting some miles on it.

[Flying Hamster]

Never got a lean/rich or misfire code. The truck started stumbling hard and I pulled over and checked, pulled the wires away from the manifold, separated them and drove home.
Swapped wires and the next couple days it would have trouble going much over 2500 rpm. Then, it backfired a couple times and now runs fine except the Idiot Light and an obnoxious rattle at idle from the cats. You can bang on the cats and duplicate the rattle, so I figure the cats melted down.

[Pyro]

Originally Posted by Flying Hamster

Never got a lean/rich or misfire code. The truck started stumbling hard and I pulled over and checked, pulled the wires away from the manifold, separated them and drove home.
Swapped wires and the next couple days it would have trouble going much over 2500 rpm. Then, it backfired a couple times and now runs fine except the Idiot Light and an obnoxious rattle at idle from the cats. You can bang on the cats and duplicate the rattle, so I figure the cats melted down.

Yup sounds like they came apart. Only fix at that point is to replace them, unfortunately.

[Flying Hamster]

O2 sensor head melted after the cat...
Must have gotten hot.