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What Tests Should We Do To Find Root Cause...

outnumbered

By outnumbered
in Ask A GM Technician

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outnumbered Enthusiast

outnumbered

Posted
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Ok, I'm back again with another emmission related code. A brief history if you haven't seen my other posts. I bought a used 2002 Yukon XL Denali about 4 months ago. First problem was low coolant light came on day after I bought it. Traced that to leaking water pump, so I replaced it. Next the truck started sputtering really bad on a long roadtrip. Had it looked at, mechanic changed fuel filter, said it was the fuel pump. Wanted second opinion, so took it to my local dealer. They scanned it, said it was the MAF sensor. Replaced it and another fuel filter. Made it 20 miles before another SES light came on. Started sputtering really bad to the point where it would not even drive. Made it home at 5 mph for last 2 miles. Started it up next morning to take to shop and ran fine. Shop says it had codes for crank position sensor and right bank cat. That cat was replaced 10 months prior, so it was under warranty. They replaced the cat, and now after about 150 miles, the SES light came on again, now for a left bank cat. For history purposes, this truck had both cat's replaced at different times in 2009. I plan on changing the Crank position sensor, but my real question is this: Before I get pissed and trade this POS in and lose my A**, what tests should I ask the shop to run. I did some reading on what can cause a P0420 code, and there are a few: Exhaust leak, coolant leak, bad o2 sensors. All of which can be tested. Anyone have any suggestions on where to start? I don't have a lot of money to throw at this truck, and I really don't want to keep throwing what little I have in more parts for it that don't fix the problem. BTW, as said truck is a 2002 Yukon XL Denali with 6.0 and 75k miles.

tylerjenkins92@aol.com Senior Enthusiast

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Well, an easy way to tell if it's a cat is to unplug then unscrew the upsteam oxygen sensor. If it's actually plugged, then the little hole where the o2 sensor was will relieve the back pressure and make it run better. Unplugging the o2 sensor and leaving it out awhile may cause an o2 code, but the truck should run just fine without it. If that doesn't seem to make it any better, then it's not the cat. Usually if your cats are going bad this fast, then you need to replace your two upstream o2 sensors, as they're reporting the wrong fuel ratio and burning up the cats. DON'T fool with the downstream sensors, replacing those are a waste of money, IMO.

cwendel4 Enthusiast

cwendel4

Posted
Posted
Well, an easy way to tell if it's a cat is to unplug then unscrew the upsteam oxygen sensor. If it's actually plugged, then the little hole where the o2 sensor was will relieve the back pressure and make it run better. Unplugging the o2 sensor and leaving it out awhile may cause an o2 code, but the truck should run just fine without it. If that doesn't seem to make it any better, then it's not the cat. Usually if your cats are going bad this fast, then you need to replace your two upstream o2 sensors, as they're reporting the wrong fuel ratio and burning up the cats. DON'T fool with the downstream sensors, replacing those are a waste of money, IMO.

 

Also, you will have a rotten egg smell coming from the exhaust that will id the problem as the cats. The smell will be greatest when the truck starts to stumble. Clogged cats also are not usually completely clogged, light throttle should let you be able to get going. Talk from experience on a carburateed car.

Also check to see if the fuel pressure regulator is leaking, just pull the vacuum line to it and see if gas comes out, that is another possibility tot he stumble.

 

Last, you can use a good old fashioned vacuum gauge to check for a clogged cat. If a cat is clogged, the reading should drop gradually at idle. Best way to help check is measure the vacuum when the engine is just lowers the idle during warm up and compare it to when the truck starts to sputter.

 

One other possible cause. Had this on an 85 fullsize blazer, the computer went bad but only caused problems when the engine warmed up. That truck also had a vacuum leak that may have lead to the demise of the ECM.

 

Check the vacuum first, then think about what else it could be. A good mechanic should be able to identify the problem and hold the dealer responsible for changing that had no issue with what was wrong.

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