Heater blows hot for about fifteen to thirty seconds when first turned on and then goes lukewarm to cold

Hello all,

Our heater blows hot for about fifteen to thirty seconds when first turned on and then goes lukewarm to cold. The heater works almost always when all controls are off with the temperature set to max hot with the outside air coming in (this is not a dual temperature control).

Recently, the heater worked great for a few minutes and then went cold again once I cycled it to cold and back to hot.

I topped off the coolant and checked for leaks. No issues.

I checked the heater core lines (the line on the left side (driver's) is hot to where you can't keep your hand on it due to the heat. The right side line is hot/warm (you can keep your hand on it without it burning).

I examined the blend door actuator right under the dash (easiest one to get to) and it is turning the plastic door shaft from hot to cold. It makes a very slight clicking noise or you can just hear it working? Is it possible for the door position sensor to be faulty even with the actuator turning the shaft?

Could my issue be from any other blend motors?

I am going to try replacing the cabin air filter, flushing the heater core, and burping the heater core as well.

I am not the best in automotive repair so please bare with me if I was confusing at all. Any help will be appreciated.

Thanks to all for the help and this awesome website!