GMC Repair: heater/AC controls 2001 GMC PU, gmc pickup, temperature valve

Question
I have a 2001 1/2 ton GMC pickup: on the temperature control the ac works great but the heater is just getting warm, back in the spring when it warmed up and I first turned control from hot to cold it didn't get cold so i put it back to hot and pushed on the knob when i turned it to cold and it worked(got cold). This winter when it got cold I have tried to turn it to hot but it is barely getting warm. i was wondering if the control knob could get stripped out or if it was electrical. Thanks for time

Answer
Hi
If it is getting warm, but not hot, I would first check the cooling system, and make sure there is no leaks, low coolant. I would also check the operation of the thermostat.
If this does not resolve the issue, I would then check the temperature valve actuator ( located under the dash) and make sure it is travelling fully from hot to cold.
USUALLY these default to either hot or cold, so there would be no temperature change when turning the temp. knob if the actuator was not working properly.