2000 K3500 question

Brav302 · Nov 29, 2022

I recently purchased a 2000 K3500 with the 7.4 Vortec. After getting home and doing a more in depth check out of the whole truck I noticed it does not have catalytic converters, and only has two O2 sensors just aft of the manifolds on both sides.

I’m pretty sure originally the truck would have had converters, and O2 sensors post converts. Is this correct? Also there are no codes or check engine light, so would you assume there has been some kind of tune done? Thanks

caw_86 · Nov 29, 2022

Nad_Yvalhosert · Nov 29, 2022

Most likely... To know for sure, pop the hood and look at the emissions label. If it says CATALYST then someone had been creatively modifying it in the past.

b454rat · Nov 29, 2022

Could be tuned or could have non foulers in place.

BeXtreme · Nov 29, 2022

As others have said, you need to look at the original emissions label. I think some of the heavy duty early OBD2 trucks didn't actually have codes turned on for the second sensors to begin with, so they wouldn't throw a code even if removed. If this truck was a federal emissions, high GVWR truck, it might not have had cats from the factory.

b454rat · Nov 29, 2022

I have 2 "heavy duty" 2500s, both big block, both have cats. I've never heard of anything after the mid 80s not having cats. Don't have to tune or do anything if put the non foulers in. I did this to my old Tahoe, check engine light never came back on...

GoToGuy · Nov 29, 2022

Nope, unleaded gasoline, with two converters, 4 each O2 sensors. First set of O2 sensors or pre cat monitors fuel mixture. Second set or after cat O2 sensor monitors cat health.
So you can find what's missing, between the emissions labels under hood and using your vin at " gm parts giant " you can figure out what's been removed if you have no vehicle history.
Good luck!

GoToGuy · Nov 29, 2022

And the last year NOT having a cat converter was the "C/K 3500" in general pickups. In 1987 all pickups 3500 series and less have cat converter's and fuel injection.
The 86 dually was the last year for regular leaded fuel and no nozzle restrictor in fuel tank.
And yes I would dump leftover chainsaw mix in then pump 10,15 or 20 gallons on top. Easy with dual tanks. That 454 didn't care.
One day my mother wanted the two row field disc moved. So I inserted a pin hitch hooked the disc up and pulled it with the dually. My dad thought it wouldn't, it didn't even grunt just walked away, like no problem. My mom laughing at my dad.

Brav302 · Nov 29, 2022

Definitely says CATALYST on the sticker on the breather. I’ll take a closer look, but I did not see any other O2 sensors, just the ones near the manifolds, so I’m leaning towards a tune.

What would be the best avenue to take a look at the tune, and see what else they may have done? The EGR and evap are in place, but doesn’t necessarily mean they haven’t been disabled. Also the speedometer is like 15 mph too fast, so I wonder if it was tuned for larger tires, and they put stock tires back on. Thanks for the replies.

b454rat · Nov 29, 2022

The down stream O2s can be tough to see, one is tucked up behind the frame, the other is underneath pointing towards the outside, AKA hard to see. AFAIK only way to tell if it's tuned is take to someone that can tune it, guessing once he plugs it in can see what's going on...

2000 K3500 question

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