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Old November 22nd, 2009, 06:16 PM   #1
JohnL
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Flashing check engine light

My brother has a 2000 Silverado Z71 with the 5.3L and whenever it gets on the highway or up over 55 for a few minutes the service engine soon light comes on and flashes repeatedly until it either gets shut off or about 10 minutes after getting off the highway.

I drove it and did not notice any kind of miss or anything. The OBDII won't pick up any codes after I shut it off. I don't have access to a TECH 2 and he doesn't have the money to take it to a dealer or shop.

When my blazer did this I finally got it to hold a code and it was a random misfire code. Thought this may be the same thing hoping someone can enlighten me.

Thanks in advance ,
John

Last edited by JohnL; November 22nd, 2009 at 06:54 PM.
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Old November 22nd, 2009, 06:19 PM   #2
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sounds to me like the cats about ready to take a shit. drive it at night and see if the cat is really hot enough to turn red.
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Old November 22nd, 2009, 06:22 PM   #3
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typically a flashing SES light means the emissions system is compromised severely and or there is possibility of engine damage. i had a 2000 sierra 4.3 2wd with the same thing happen except it had a horrible missing under heavy load and severe spark knocking. turned out the engine had a faulty plug wire for a long time causing raw fuel to build up in the catalytic converter plugging it up solid. it needed a whole new y pipe with new converters, and a fresh tune up to fix the problem.
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Old November 22nd, 2009, 06:34 PM   #4
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any time the ses flashes it means there is a missfire detected seen the exact same thing kodiak450r said a few times he prolly set you on the correct path
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Old November 22nd, 2009, 06:54 PM   #5
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Quote:
Originally Posted by Kodiak450r View Post
typically a flashing SES light means the emissions system is compromised severely and or there is possibility of engine damage. i had a 2000 sierra 4.3 2wd with the same thing happen except it had a horrible missing under heavy load and severe spark knocking. turned out the engine had a faulty plug wire for a long time causing raw fuel to build up in the catalytic converter plugging it up solid. it needed a whole new y pipe with new converters, and a fresh tune up to fix the problem.
I will have him take it out tomorrow night and see if it is heating up. Thanks.
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Old November 22nd, 2009, 07:11 PM   #6
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My cousin had a 2000 or a 2001 and when his did this it ended up being the catalytic converter. Dealer wanted $800 to replace it. The dealer cat was over $400 itself....
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Old November 22nd, 2009, 07:14 PM   #7
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If the cat is bad I will just buy him a high flow cat and a muffler for christmas and do his exhaust for him. Cheaper and better than just buying a new stock one.
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Old November 22nd, 2009, 08:04 PM   #8
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Hook it up to a scan tool to see if it's a generic misfire code (300) or specific to a cylinder.

Hopefully its one cylinder, and then you can check that cylinder for spark, fuel, and worst case, compression.
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Old November 22nd, 2009, 08:09 PM   #9
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just had the same problem with my truck ended up being a spark plug wire.
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Old November 22nd, 2009, 08:16 PM   #10
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Flashing = misfire as previously stated.

With it not holding a hard code I doubt its a wire. With it acting up on the hi-way it is most likely a cat plugging up. Any scan tool that can give data should be able to read misfire as it occurs (being a GM). Drive it and when it starts flashing see what cyl's are recording the misfires. If it is specific to one side of the motor on more then one cyl then its most likely a convertor.

Look at your exhaust manifold bolts. The GM V8's seem to break (yes break) the rear ward most bolt on the manifold. This then allows the computer to get a poor reading on that bank which alters fuel output etc and plugs the convertor for that bank over time. The broken bolt is 99% easily removed once the manifold is off, you can grab the bolt with a pair of vise grips or worse case weld a nut to it and turn it out.
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Old November 22nd, 2009, 08:52 PM   #11
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Check TSB Ref # 06-06-04-046
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Old November 24th, 2009, 02:15 PM   #12
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The cat was plugged. Replaced the cat and now it is running better than ever. Thanks alot for all your help guys !
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Old November 24th, 2009, 03:45 PM   #13
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Did you check the manifold bolts
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Old November 24th, 2009, 03:46 PM   #14
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Quote:
Originally Posted by CheapThrillB2 View Post
Did you check the manifold bolts
Yes sir. None were broken and seemed to be in good shape. Thanks.
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Old November 24th, 2009, 09:43 PM   #15
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Quote:
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The cat was plugged. Replaced the cat and now it is running better than ever. Thanks alot for all your help guys !
don't overlook the possible underlying problem though. a catalytic converter should not go bad unless there is an engine problem to cause it. many times i've seen people replace converters on cars that had an underlying condition for a period of time that caused the converter to go bad. not the other way around.
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Old November 24th, 2009, 09:45 PM   #16
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Quote:
Originally Posted by Kodiak450r View Post
don't overlook the possible underlying problem though. a catalytic converter should not go bad unless there is an engine problem to cause it. many times i've seen people replace converters on cars that had an underlying condition for a period of time that caused the converter to go bad. not the other way around.
It did trip a misfire code also. We bought all the stuff for a tune up also and hoping to knock it out this weekend. Hopefully it will not have any other issues.
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