Lack of heat in Nissan Frontier 2004 w/v6, air, automatic 4x4

I only have luke warm air coming into my truck, and I was wondering if anyone else might have had the same problem and solved it. The switch on dash switches from defrost to floor or thru dash . Was thinking maybe a switch under dash not working maybe, but do not know where to start looking. Would appreciate any input I can get, thanks.

There is a blend door that regulates the mix of air for temp control. Most are little motors that fail. Older models use vacuum. The other problem could be air in the system if it was not bled properly after draining Or you have a leak. Are you loosing fluid?

Is the engine getting up to operating temperature?

Is the thermostat working properly?

Are both heater hoses hot?

If only one of the heater hoses is hot, you might have a partially plugged heater core

Is the temp gauge showing normal coolant temps, or is it hovering near the cool zone? A stuck open thermostat may be preventing the engine from warming up fully. the coolant doesn’t heat up, and there is no heat for the cabin heater.