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P0420, might it be another exhaust leak?

7K views 14 replies 5 participants last post by  repairman54 
#1 ·
I developed engine code P0429 today. Might I get lucky and it’s just the 4th exhaust manifold? I changed the o2 sensor last go around because it looked deformed. I also painted the manifold in hopes it would protect it from cracking due to external causes. It’s been sounding bad again, might a crack actually cause this code, id say it’s not the newer ac delco sensor. I doubled checked today and it’s 139k and my oils consumpetion was at 112. I’ve been off on mileage in the past because well it’s unreasonable to have so many failures in so few miles
 
#3 ·
To protect not prevent, the others tended to surface rust and it seemed to me it might prevent futer weak points though mine seem to crack in the same spot.

So can a bad enough split cause this code?
 
#4 ·
The exhaust manifold has a heat shield, are you putting it back on after replacing the manifold?
P0429 is a generic code, I don't see that one in the GM Service Manual.
P0420 is usually a catalytic converter code.
 
#6 ·
Sorry it was a typo, I will go back and edit it for clarity. Code P0420 is correct.

Yes, I have reinstalled the heat shield each time. I do have another horrible sounding exhaust leak. I am really asking if enough unmetered O2 could be introduced by a crack in the manifold to cause the O2 sensors to read differently enough to produce the error code. I previously had caught that the wiring to the my downstream O2 sensor was not pinned properly to the engine and I corrected that before any fraying could actually occur to the other sheathing. Thus I believe my O2 sensors to be functioning properly. I want to say that about 6000 miles ago (two oil changes) I paid for the dealership to diagnose a misfire and they stated it was fine. I know misfiring and oil consumption can contribute to this code appearing and it seems like a real money pit unless I just got yet another poor quality cast header. As I stated, I painted the header this time around to try to prevent any external forces (corrosion) from shortening its lifespan. VHT exhaust paint/primer was used. I also changed the exhaust manifold flange to catalytic converter flange studs to grade 10.9 bolts to aid in removal of the header in the future.
 
#7 ·
Still have this code at 167771 miles. Have not changed any parts. Getting to where I want to get this fixed in order to get the car marketable. Can anyone interpret my scanners reading? One odd thing is during At least 2 of the freeze shots I was going 55 and it says 0mph.
17414
17415
 
#9 ·
#11 ·
What is the torque for the catalytic converter to exhaust manifold flange? One gasket failed after approx 500 miles and the second one 350 miles. Installed catalytic converter from eastern catalytic converters. The p0420 cleared up until the gasket failure. Now I have p0420 and p037. One the second gasket I went just past wrench tight and tried to retighten here at my destination, now it’s sounds worse. Long embarrassing 400 mile trip home to come. No way to lift it up to take the support loose beneath the flange to change the gasket.
 
#13 ·
Leave the heat shield off.
My theory is the heat is trapped under the heat shield and can't dissipate.
The excess heat is causing the manifold to crack.
 
#14 ·
I pressed this manifold up using the Chris fix method. Luckily this last one isn’t cracked. Did you relocate the wire harness running aft and just above the manifold. I’ve patched my loom there, there’s a lot of heat coming off that manifold. Is that why Toyota’s are in the front?
 
#15 ·
Engine design is the Toyota difference. Lots of weight hanging off that manifold. 2.4 in Malibu's also have cracking issues. Crappy design with thin materials is the issue.
 
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