The heater is not working and the fan only blows air on high.
Hello. There are several things that could cause that heater to not work properly and the fan to only blow air on high. It is possible that you have a bad thermostat, which would cause the lack of heat. It is also possible that your heater control valve could be malfunctioning or that your heater core is going bad or is clogged. The most likely cause for the blower fan only blowing on high is that the blower motor resistor is malfunctioning. To determine the proper fix for your truck, a certified technician, such as one from YourMechanic, should diagnose your heater issue and make a recommendation for proper repairs to get your heater and fan working properly again.