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Bill, Chevy Technician
Category: Chevy
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Experience:  GM Certified Master Technician
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2003 Chevy: accelerate..check engine light came..Mass Air Flow

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My 2003 Chevy Avalanche with 135K miles on it start running rough yesterday. It does ok at low RPM and low speeds, but struggles when you try to accelerate. The check engine light came on and AutoZone said it was the Mass Air Flow sensor. I used a MAF cleaner on the old one, it didn't work. I installed a new MAF, still didn't fix the problem. Still running rough. What do you think it is?

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This is very likely a restricted catalytic converter. What happens is the restriction in the exhaust causes incorrect airflow through the engine. The air can't come in because it can't get out of the exhaust. Therefore the readings the mass airflow "should" see related to engine vacuum, rpm and load are slewed off from where they should be. This often shows up as a mass airflow sensor fault code even though the restriction is in the exhaust. A test with an exhaust backpressure gauge will confirm this. I've seen some techs temporarily remove the oxygen sensor and drive it (albeit very noisy) to see if there is a difference in power. Based on your description I'm as confident as I can be from my chair that a restriction in the cat is your problem. I would suggest however confirming it either way suggested above as they can be a bit expensive and we want to be sure. :)



Customer: replied 6 years ago.
Thank you for your help and advice. Why do you think this problem came on so quick? Seems to me a restriction in the Cat would build up over time. Also, I'm not afraid to take on some repairs. Is this a repair that a mechanically inclined homeowner can accomplish, or should I take it to a shop? How many hours does Mitchell or AllData call for it, and does it require any welding?

It often happens that way. There may be a partial restriction for a while but eventually it just gets to the point where it affects engine operation. Generally you're looking at an hour and a half or so plus any diagnostic charges or broken bolt charges. Again... I strongly suspect that's your problem but please do confirm it with the backpressure test or the other way. Replacement of the cat at home is not always the easiest thing to do for two reasons. One is the angle it needs to be moved to in addition to moving the trans crossmember is all but impossible on the ground. Secondly is the nuts/studs that hold the converter onto the manifold quite often are frozen on and may require some creative measures to get them off... sometimes air chisels... torches, etc.

Customer: replied 6 years ago.
Understood, I will enlist the help of an expert. I may however try your described testing method of removing the oxygen sensor to confirm the problem. Considering this may not get fixed today, how do I reply when the problem is fixed to confirm your suggestion was correct, and give you positive online feedback?

You merely log in here and click on your questions. I'll be here. :)


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