96' Honda Accord P0420 -- Leaking exhaust manifold?
 

Back in May, 2011 we took off the exhaust manifold to replace the gasket. We also replaced the o-ring.

We also took it off to replace one of the broken off studs. My dad drilled it out and supposedly "re-tapped" it. Well, the bolt fell out, so much for that.

I probably should have had it professionally done(too late for that, only constructive feedback please) And I see black soot around the broken stud we drilled out. That's got to be tripping the P0420 code. Which comes on between 1 -5 days after I clear the code.

I know for a fact the upper stream 02 sensor works because it fell out numerous times while driving and my car screeched to a brisk jog! It could possibly be the downstream 0^2 sensor, but I want to fix this before this May when I have to have my car inspected again.

Has anyone sealed a leaking upper exhaust manifold before?

How did your father do the repair the first time?

He drilled out the bolt into the block and re-tapped it at 5/16''.

(if only someone would have uttered the words stud extractor...)

You might want to look into a helicoil. 5/16 might be the right thread pitch to install the helicoil and match the original thread pitch.

The helicoil should fix the leak when done right thay are great. Now on your code P0402 that is a bad cat the ECU looks at the O2 senser in the cat and can tell when the cat is not working I just rep;aced mine with a NAPA unit fit great and so far so good stay away from one size fits all Get the leak fixed first becous nothing will be right when it is leaking

Sorry I got it wrong, the bolt is actually still in there. Aluminum block...steel stud. I'm not sure even if a professional could save it. They'd probably recommend replacing the block. Okay, that's happening.

To test how bad the leak was, I had my dad put a rag on the exhaust pipe and I opened the hood and I saw some stuff fizzling out where the gasket was black(on the upper exhaust manifold). It's actually didn't fizzle where the bolt was missing, it fizzled some towards the center and I torqued those bolts down a little bit.

Code situation: the code come backs every 2-5 days, so I think the catalytic converter is still hanging on a bit. I could try to take it off myself, it's actually the bolt on but the bolts are rusty to kingdom come. But I'll probably have a muffler shop put a new one on.

Do the exhaust leak first. O2 sensors actually measure oxygen DIFFERENCE outside vs. inside. Exhaust blowing onto the outside of the sensor will throw off the measurement.

Exhaust blowing onto the front O2 sensor will throw off the fuel mixture. That in turn can lead to exhaust composition that can't be properly handled by the cat. That in turn gets measured by the rear O2 sensor & it throws P0420.

Well, I'm trying to trouble shoot what shot my catalytic converter or what's taking it out.

It doesn't look rotted out, it doesn't rattle and there's no sign of rotten eggs.

Could it be the spark plug wires? How do I test these?

The scenario I said about exaust leaks, doesn't destroy the cat. It simply runs the engine at the wrong mixture, so the rear O2 sensor THINKS the cat isn't working.

Don't be too tough on your Dad. I had a broken manifold stud too, and no amount of left-hand drills/stud extractors/cuss words would get it out. I ended up drilling and tapping an 8mm hole and putting a new stud in...slightly off kilter, but it worked.

5/16" is slightly smaller than 8mm, so there's a possibility that you can re-tap the hole for an 8mm stud. Otherwise, Helicoil. $23 for the kit at Amazon and, depending on which stud, it should be pretty easy to do.