2000 GMC Sierra 1500 problem

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Hello to the group,
I am having a problem again with my service engine light coming on. Over the summer I posted a note to this group about my truck being difficult to start
once it was warmed up. It was the fuel pressure regulator as a few people here told me. We fixed that.
Now it is a more difficult problem. The 'service engine light' is on. I have tightened the gas cap (I even replaced it), and the other day my mechanic put it on his analyzer and reset it. He said he thinks it's the O2 sensor this time. But I thought there was more then one O2 sensor (?). Anyway, he made the light go off, but after I drove about 10 miles, it came back on again. It is a 2000 GMC Sierra 1500, 4WD, with a 4.8L vortec engine. I realize this may not be enough information, but it is all I have. The truck seems to run fine. There's no other symptoms. Just that damn light.
Anyone have any suggestions?
TIA, Joe
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Joe,
Go to Autozone, Advance Auto, Checkers, Pep Boys, NAPA, etc........ and have them scan your comnputer for FREE. They will get one or more codes from your computer. Post the EXACT codes (not the description they'll give ya' at the parts store) and we'll take it from there. The codes will look like P####.
Doc

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I didn't know about this. I can probably go to autozone tomorrow AM. I will post the codes when I get them.
Joe
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Your "mechanic" should have read the code before he turned off the light. No good just guessing what the problem is when the code will narrow it down. In the case of an O2 sensor it would identify which bank and which sensor. Go to a mechanic that will at least read the code before you just start throwing parts at it.
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Hi Calhoun,
He just checked it quickly the other day. I am supposed to go back and see him on Thursday. He is not the type to just 'throw parts at it'. He is an old timer and not very familiar with the computers on board our trucks, but he is learning this new fangled stuff. That's why I wanted to do my own research. I just want the repair, whatever it is, to go smoothly.
Joe
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Joe,
If he hasn't learned by now (EFI has been around for 20+ years) he's never gunna learn. I suggest you find a new mechanic.
M

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Hi Doc,
Yeah, you're probly right, but until I do, he is really the only one I know I can trust.
Anyway, I went to autozone this AM. The error code is P0430.
Any ideas on what this code means?
Joe
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A quick Google for the terms "P0430 ODBII" returned the following: P0430 Catalyst System Efficiency Below Threshold (Bank 2)
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Joe wrote:

A quick Google for "P0430 ODBII" found the following: "P0430 Catalyst System Efficiency Below Threshold (Bank 2)"
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Joe,
This indicates that the computer thinks the catalytic convertor on the right side (passenger side) is operating inefficiently. This usually means that post-cat 02 sensor on that side is shot, but it could also be a bad or failing catalyst. The repair is usually to replace the post-cat sensor on that side and see if the code goes away.
If you have smog tests where you live, you could bring it in for a pre-test and see if the sniffer is picking up higher-than-normal HC levels which could be indicitive of a failing catalyst. If the sniffer says HC's are ok, replace the 02 sensor.
Doc

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Doc,
Thanks, your explanation is much clearer than just the message that the tech showed me on his analyzer: "catalyst efficiency below threshold" Now I know why you told me to just get the error code and you could handle the rest.
We have emissions tests here once every 2 years. This is not my year to have it, but I will get it checked out and fixed anyway. I hate seeing that message. Oh, one more thing, the tech at autozone reset the computer because he said once in awhile it's just a glitch. Last week when my mech reset it, the message came on after about 10 miles. It is not on yet and I drove about another 50 miles so far today, but we'll see.
So the sensor after the cat on the passenger side is the one to replace first, if HC's are OK. And if HC's are not OK, I need to replace the cat??
Thank you again for your detailed information.
Joe
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HI, Just let me chime in again and add to Docs great info. This code is set when there is too big a difference between the pre and post sensors readings. Any leak in the exhaust system will also set this code. So check this first. It can be set by a bad sensor but this will usually set a code of its own. (0136-141). Some inexpensive readers will only display one code when you may have more than one. Your mechanic can scan bank 1 sensor 2 and compare it with bank 2 sensor 2, than switch sensors with each other. If readings are still the same than sensors are good. It could actually be a bad cat. Check your warranty most have 100K miles on emission equipment.

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Hi Calhoun,
I am close to 100K miles now. I will be seeing the mechanic tomorrow, I can relay your suggestions to him. Thanks for the additional info.
Joe
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I saw the mechanic yesterday (Thursday) and he replaced the O2 sensor per Doc's info, and, so far, no light (I have put about 100 miles on it since then). Now I found out that I am going to have to pass emissions this month (yikes!) in order to get an inspection sticker, so I will find out how the fix went.
I have another question though. I am a bit confused. When the mechanic replaced the part, he used the analyzer to reset the code, so the 'service engine' mesg turns off. He does not have a sniffer at his garage but told me to call the place where I usually get my inspection sticker because I have to drive about 300 miles to 'clear' the code??? I called the inspection place and he agreed. I am guessing, but they must check the analyzer before connecting the truck to the sniffer for emissions testing? But why isn't the code cleared out right away? The inspection place guy told me the best thing to do was put a couple of hundred miles on it, but highway miles, ie go on a run for about a half hour or more at a time on the highway before going in for the inspection. He said short trips (ie 15 minutes), and turning the engine on and off won't clear the code as fast as a couple of long highway runs.
Does anyone know about this and why?
Thanks, Joe
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Nothing will wipe out the codes and every other piece of stored information like sitting overnight with the battery cables disconnected.
Doc

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Aha! I figured there must be an easier way. But does it wipe out things I need? I mean, will something need to be re programmed, or is this a safe way to do it? I would rather not put 300 or so extra miles on my truck with gas prices the way they are...
Joe
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Your computer won't forget anything it can't re-learn in the first 15 minutes of running after a restart. If you have a theft-lock stereo, make sure it's turned OFF before you yank the cables.
Doc

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Ok, thanks Doc
Joe
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Thanks to everyone who replied. I was able to get my inspection sticker this AM. I had put a little over 200 miles on it since the O2 sensor was replaced and the code was reset last week.
Joe
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I will try to explain how some of these inspections work. First you need to know how some codes are set. To set some codes a certain amount of cycles must be run. i.e. the vehicle must be started x number of times and run above 2000 rpm for x amount of time etc. If the set number of cycles have been run when you hook up your scanner that "systems monitor" will report if the test is complete or incomplete. There are many different system monitors. To pass an inspection you can have no more than x number (3 in my state) of incomplete tests. ( this prevents people from clearing codes just before they enter the inspection.)This is why it is advised to drive 200 miles first. Odds are in that time the systems test will be completed.

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