The dreaded flashing Check Engine Light

swathdiver · Jan 19, 2018

Do the GMT800s have 4 O2 sensors? Regarding the Tech2 or a scan tool that can record, did they have the Tech2 hooked up when the CEL was flashing while driving down the highway? They could record that and review later against the trouble shooting diagram for your truck.

I suggest you get a subscription to ALLDataDIY for access to this material.

Regarding the 02 sensors, if a cat is going bad it will throw a cat specific code, telling the computer which one. The downstream O2 sensors measure the health of the cats. That is of course, if you have them.

Ohh and btw, welcome aboard Joe!

mizzouguy · Jan 19, 2018

Does your exhaust smell like sulfur? I'd also think cats but which cel codes are on would help....could also be intake leak or injectors.

Scottydoggs · Jan 19, 2018

you can also remove the o2 sensors pre cat and go for a ride, yes its loud and sounds like crap, but if the cel dont flash when wot, you just confirmed the cats are bad.

you can find high flow cats for about 50 bucks each on ebay, would hope a shop is less then a 100 to r and r the cats.

HiHoeSilver · Jan 19, 2018

Scottydoggs said:
you can also remove the o2 sensors pre cat and go for a ride, yes its loud and sounds like crap, but if the cel dont flash when wot, you just confirmed the cats are bad.

you can find high flow cats for about 50 bucks each on ebay, would hope a shop is less then a 100 to r and r the cats.

@Tonyrodz has some I think.

Tonyrodz · Jan 19, 2018

HiHoeSilver said:
@Tonyrodz has some I think.

I do.

rockola1971 · Jan 19, 2018

Id slow down a little here. You mentioned the alternator needed replaced. Why? Has it been replaced yet? If the alternator has an internal short in the diode pack or its rotor then it can load down your battery and cause every sensor you have to be supplied with low enough voltage that the whole system as a whole will not operate correctly and throw false codes or you could very well be getting valid misfire codes...because at higher rpm the ignition coils require higher amounts of current (higher duty cycle) to supply the engine with spark but cant because the whole charging system is loaded down from a defective alternator.

Motojoe86 · Jan 20, 2018

rockola1971 said:
Id slow down a little here. You mentioned the alternator needed replaced. Why? Has it been replaced yet? If the alternator has an internal short in the diode pack or its rotor then it can load down your battery and cause every sensor you have to be supplied with low enough voltage that the whole system as a whole will not operate correctly and throw false codes or you could very well be getting valid misfire codes...because at higher rpm the ignition coils require higher amounts of current (higher duty cycle) to supply the engine with spark but cant because the whole charging system is loaded down from a defective alternator.

Motojoe86 · Jan 20, 2018

Lots of great information from everyone here. I appreciate all the input. Rockola, I am actually really hoping that the alternator is in fact the problem. I didnt realize it could cause flashing cel. I actually just finished changing that out, new belt and a second clean of maf and throttle body. Im beat for the night but im gonna hop on highway tomorrow and try a couple minute run at 80 or so and see if it comes back on. I will update you guys tomorrow. Thanks again everyone for all the help. This site is awesome!

M1Gunner · Jan 20, 2018

Also if you ever think you have a cat issue all you need to do is hook up a vacuum gauge to your intake manifold. And if the cats were restricting airflow out of your engine, aka a big ass air pump, at higher rpms then the vacuum gauge would be an immediate tattle tell.

swathdiver · Jan 20, 2018

What might it read for a restricted cat?

The dreaded flashing Check Engine Light

swathdiver

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mizzouguy

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Scottydoggs

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HiHoeSilver

Away!

Tonyrodz

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rockola1971

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Motojoe86

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