04 Tahoe - Reduce engine power light comes on if I run A/C on high on hot day. This reduces the speed to max of 40mph. The coolent level is fine. After I let the truck set for a while then light goes off and normial speed is restored. I have disconnected negative to reset computer but that has not helped.

 

Welcome: What does the coolant temperature gauge read when this occurs?

 

It climbs pretty high . But the temp. does not get high enough to over heat. Could it be that the T-Stat is not working properly?

 

The "reduced power" message will come on before the engine over-temps to protect it from damage.
It sounds as though there is a problem with the cooling system of your Tahoe.

I would recommend checking the engine driven cooling fan clutch, or have a mechanic go completely through the cooling system.
It could be a faulty radiator cap, thermostat, debris or bent radiator or condenser fins, corrosion in the cooling system etc.

 

I took it to a dealer and "they" told me there was nothing wrong. It ran fine for a couple of days then the trouble started again. I will check the fan clutch