Heater not working?

Easy solution, swap the truck with someone from Alaska. :-)

Actually the problem is likely a failed sensor. It is telling the HVAC controller that the interior temperature of your truck is already over 86 degrees.
One of the common ones that fail and cause this is the internal cabin temperature sensor, it is the thing that looks like a speaker in the headliner over the drivers door.
It has a tiny fan and a thermister inside. They suck in crud and it makes them fail.

You replace it by removing the interior windshield trim on the left side, the center pillar trim and handle and then the left side sunshade. Then you gently pull the headliner down. You will see the sensor. Disconnect the wiring connector and then the interior grill comes off.

ACDELCO # 22834330

I need you to check a couple things. This guide will help

https://www.2carpros.com/articles/car-heater-not-working

Let me know what you find.
Joe

I have attached schematics for you to review. The GMC model uses electric and not vacuum for some reason. Review the schematics to see if you can identify where to check under the dash. Also, as far as the high speed, it believe it is a relay rather than the resister. See schematic.

You need to open it up and investigate why-could be a door or an actuator problem-

Have you replaced the thermostat? Is the coolant full? As far as the heater, with the engine running, hot, and the heater on high, do both heater core hoses get hot?

One last question, is the blower motor working for the heater or is that the issue?

Check to see if your actuators are getting power or not. You may have a control head going bad. Also cleant eh battery cable ends as it may be making poor contact until you get more revs going to put a better charge int ehbattery. You might also check your battery, most auto parts stores can do this for free. I've included a generic pic of actuators.

Good evening.

What are the temperatures of the heater hoses under the hood?

Can you hear the blend doors moving when you command heat and cold?

Roy

https://www.2carpros.com/articles/replace-blend-door-motor

Okay, thanks.

It sounds like you have a clogged heater core. The temperatures are too close. There is usually a bigger difference it temperatures.

Try removing the hoses and flushing the core with a garden hose. Flow it back and forth several times and see if get debris out of the system.

Roy

Keep us updated. Feel free to ask any follow up questions.

Roy