Cabin heat and defroster do not work correctly at highway speeds

Very little air from Defrost? Floor? Panel Vents? With it on Inside Air position? Does Blower work on all speeds? Let's start there.

Cabin air filter? Blower motor working on all speeds? Critters building nest in heater box

Hi Cheri, do you mean the fan is not blowing air out of the air vents?

if so could be one of three problems;
(1) blown fuse on a/c heater circuit.
(2) sized or faulty blower/ fan motor.
(3) faulty fan speed resistor.

Plugged cabin air filter ? Mode or temperature door stuck closed ? Can you change direction of air flow ? floor ,vent , defroster . Change the heat to A/C .

check the mechanism that controls the heat flow .The flap may not be opening fully thereby ,impeding the full flow of heat into the cabin. .

Try hitting the blower motor with a hammer handle and see if it starts, if it does replace it.

Change engine cooling system thermostat.

Hi Espd, The heater inside the cabin is a very simple affair. It circulates the coolant from the engine cooling system through small radiator(s) inside the cabin. As the system works at high RPM it sounds to me as if there is a restriction in the system. The restriction could easily be a defective mixing valve or a clogging of the system. As your vehicle uses an electronic climate control system, it may well be a problem in that system. Have a diagnostic scan carried out on the climate control. If faults are recorded have them attended. If not, back flush the cooling system. (Back flushing as you know, means flushing the system in the opposite direction to the operating flow). Check the operation of the valve and make sure it is opening fully. Regards John

1-your thermostate is stuck open half way. thermostate is a simple device that made of enlarging metals when heat is applied. it suposed to be in close position to allow engine heat up, when specified temp reached it opens and let it flow and radiator tries to keep coolant cool.when it is broken it stucks or it acts slow or it wont acts at all. in your situation it is stuck open half way. it allows enough water to be heated but flow is limited to heat cabin heater.

2-if there are cabin heater selenoid/selenoids for two different temperature for cabin left and right, on some instance those selenoid valves to adjust cabin heater stuck open or closed like thermostate acts on first case. so heat is not enough to heat up cabin heater.

Sounds like one of the fancy heaters. They have circuit boards that can go bad. don-ohio