03 Silverado issue I cant figure out

[TJ Olivia Barnes]

Hello everyone!! I have an 03 silverado 4.3 votec 134k miles I got a year ago and its my first Chevrolet. I have an issue with misfiring around 1200-1600 rpm (usually overdrive) and i cant figure out the issue. I replaced the fuel filter, ignition coils, cap and rotor, plugs, and wires. I get a cat below threshold code, misfire cylinder 6 or multi cylinder code. The truck runs rich, it has a black buildup on the exhaust tip. I'm thinking tps because it seems to occur when the throttle would be pressed when normally cruising down the road maintaining speed. The trans shifts funny at times also leading me to believe tps even more. I wanting to get your guys take on it. Thanks in advance.

[TJ Olivia Barnes]

update: Cylinder 6 misfire detected today.

[DaveH71]

Very rarely do i see a catalyst efficiency below threshhold code that isn't the cat itself.

 

I would start looking at the cat or cats depending on your model.

[TJ Olivia Barnes]

The cat code itself isnt bothering me unless that would cause a misfire but i wouldnt think it would. they have a little rattle to them. If they could cause the misfiring though I'll cut em off tomorrow, no inspections or anything here in tn.

[TJ Olivia Barnes]

Update: I replaced the cats and no change. Still missing when cruising w/ light throttle. No code yet but very noticeable when going down the highway with hills. It misses a few times and shifts up. At times it missing on flat road, popping the throttle to the floor solves the issue for a couple miles. Any ideas? Changing the tps maybe?