Heater only gets warm when fan is turned to High, anything less than high, heater blows cold air.
Can we assume that you already checked the coolant level?
Yes, coolant level is fine.
any recent cooling system work done?
Does the truck have a temp gauge on the dash? Is it in the proper range?
The basic parts must be working, otherwise there’d be no heat on the high fan setting either. For heat to blow out the vent at fan settings other than High.
- coolant must be flowing through heater matrix
- fan must be spinning
- pathway from fan through matrix and out to vent must flow air freely
LIsten for fan. If it is spinning at what seems like the correct speed, I expect the problem is the last one on the list. There are usually several plastic doors that have to get in the correct configuration for the path to flow air freely, and likely one of those doors is in the wrong position. Could be stuck from accumulated gunk in the hvac unit, door or hinge broken, open/close actuator faulty, electronics problem preventing solenoid from actuating. I"m guessing it is a door-position actuator has gone south. If so, some are relatively easy to replace, and some definitely are not easily replaceable. For your wallet’s sake, hopefully it is the former.
Good idea to check if other posters here have reported this same thing. Forum search feature link upper right this page.
One other idea, sometimes the hvac actuators get confused what position they are actually in, and require a reset procedure. Shop w/Titan experience would likely know how to do that.