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Pete
Pete, GMC Technician
Category: GM
Satisfied Customers: 29085
Experience:  14 years GM dealership technician and training.
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2003 yukun denali 6.0l, multple misfire bank 2.

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2003 yukun denali 6.0l, multple misfire bank 2.
Hello,I will be glad to try and assist you with this,Welcome to Just Answer.

Hi.

The most common issue with a lack of power and misfires on one bank of cylinders is the exhaust catalytic converter will plug up and causes excess back pressure which causes one bank of cylinder so misfire like this.

The exhaust back pressure needs tested to see if it is plugged.The back pressure shouldn't exceed 1.5 psi through all rpm ranges.If the passenger side(bank 2) tests over this spec then the cat is plugged and needs replaced.This is common to fail like this and cause this problem here as I have had to replace the catalytic converters before for this failure.

Any exhaust shop,shop or dealer can do the back pressure test to see if it is plugged.From what is described most likely this is the source of the failure.

Let me know if you need anything else,click accept if you feel satisfied with your answer as this is how I get credited for my time,Thanks Pete!
Customer: replied 6 years ago.
I will try checking the cats this evening. But I thought if the cats were colapsed it would give an emmissions related code instead of a fuel and air related code.
No,.In most cases it will give misfires like this and cause this exact condition here on just one bank as the engine has two cats one for each bank.I have ran into this being a problem quite a few times before and replacing them corrects the condition.I would still have the passenger side (bank 2) cat back pressure tested to see if it is restricted.

In some cases it can set a code for being restricted but I have seen quite a few that didn't set any codes except the misfire codes.

Let me know if you need anything else,click accept if you feel satisfied with your answer as this is how I get credited for my time,Thanks Pete!
Pete and 6 other GM Specialists are ready to help you
Customer: replied 6 years ago.
Thanks for the help Pete. I took the exhuaust pipe loose from the manifold . The truck ran great. I removed the cat and replaced it with a new one. Problem solved thanks to your great knowledge of gmc trucks. Anthony Maddox
Thats great to hear.I am glad to hear it is running good again.

Let us know if you need anything in the future.

Thanks Pete!