Any suggestsions on where to look first? I’ve checked radiator fluid. The AC works great. There is very little heat coming out when I set the intake to external, have the fan set low, and am driving faster than 50 mph.
Is the engine warming up? Are the hoses to the heater core hot?
A stuck thermostat is a common cause of little or no heat. Try a new thermostat.
If there’s a valve that controls coolant flow to the heater core, make sure it’s working.