The reduce power light comes on and the check engine light stays on what may cause this problem

Hook up a professional grade scan tool to the vehicles computers an check for DTC'S diagnostic trouble codes , that will tell a professional mechanic what to check.

When the check engine light comes on it disables other module features as a safety issue. The reduced engine power light is normally caused by a defective throttle actuator motor assembly. In some instances, it can also be caused by a defective gas pedal sensor as well. In my experience, the throttle actuator motor is the most common repair

Jerry, Drive to a good shop. So many things can cause reduced power light unfortunately. The first step would be to get the truck hooked up to a scanner/computer and have the computer scanned for trouble codes. One thing that you could try is to drain the water from the water separator that is mounted on the frame under drivers front door. If the PCM picks up on water, power is reduced until water is removed/drained. It could be an exhaust sensor that is reading faulty. Unfortunately from here its impossible to tell you exactly what the fault is because there are just so many engine sensor inputs that could cause your problem. Check the attached links, forums,instruction and guides, Good luck
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Help reduced engine power light comes on

Run a diagnostic test to see what codes come up and go from there.

sound like you have a co sensor problem. you will have to take it for OBD scan. will give you the error code for what sensor is faulty.

Engine power reduced , problem with the electronic throttle control ! Watch a couple of video's on youtube , google GM engine power reduced . The gas pedal an the throttle body have position sensor's ,when the PCM/ECM - engine computer see conflicting input's from one of these sensors it send's into a reduced power mode . Your best bet mite be to take it to a qualified repair shop.

You may need to find a different shop

Nothing showing up, is about as common
as any vehicle coming into a repair bay

They know you won't pay diagnostic time or believe
anything your told,so they sent you on your way

you need to scan you vehicles ECM, there is a trouble code stored there. depending on where you live most parts stores( such as a shucks/orielleys) will have a scan tool they will let you use if the vehicle is in thier parking lot. this should diagnose the issue, provided the battery hasnt been disconnected since then.

need to have vehicle scanned the trouble codes will be stored in the computer most auto stores offer thi service for free check ur local area there are a few hundred different codes or problems that can cause ur problems and being interment can also be an electrical connection/harness problem

This has nothing to do with the heat on the vehicle .
Reduced Power
The reduced power telltale is turned ON by the cluster when it receives a command from the body control module (BCM). The engine control module (ECM) (L81) engine sends a message to the BCM over the CAN serial data link.
The reduced power telltale is illuminated when the ECM is limiting engine torque in a low traction situation. This telltale is used only for the V6 (L81) engine application.
The reduced power telltale is turned ON by the cluster for 2-3 seconds, when the ignition is turned to RUN, for a visual system check.

Taking the vehicle to a qualified repair shop an having it diagnosed would be your best . Unless you have a factory tech 2 scan tool to read BCM DTC'S - diagnostic trouble code's an have factory diagnostic repair info.
Reduced Engine Power Mode
When the ECM detects a condition with the TAC system, the ECM may enter a reduced engine power mode. Reduced engine power may cause one or more of the following conditions:
• Acceleration limiting--The ECM will continue to use the accelerator pedal for throttle control; however, the vehicle acceleration is limited.
• Limited throttle mode--The ECM will continue to use the accelerator pedal for throttle control; however, the maximum throttle opening is limited.
• Throttle default mode--The ECM will turn off the throttle actuator motor and the throttle will return to the spring loaded default position.
• Forced idle mode--The ECM will perform the following actions:
• Limit engine speed to idle by positioning the throttle position, or by controlling the fuel and spark if the throttle is turned off.
• Ignore the accelerator pedal input.
• Engine shutdown mode--The ECM will disable fuel and de-energize the throttle actuator.