How do you repair a heater always stuck in the on position on a 1998 Mazda B2500 pickup truck?

Answer 1

Your model should just have coolant flowing directly into the core at all times. A simple check of the flow is to feel both heater hoses going into the firewall from a cold start. The hoses should warm up at about the same rate and get uncomfortably hot. This is a good indication that heat is getting into the passenger compartment.

If it appears that the core is getting hot, the next suspect is the blend door. This door diverts some or all of the air flow through the heater core and commonly fails on the Mountaineer/Explorer/Ranger/Mazda pickup...all of which use the same plenum box. The door is on a vertical axis and has a pervasive problem with the the plastic connection to the motor breaking. The door can swing back and forth randomly or wedge in a strange position, killing heat and AC efficiency. This is an expensive dealer repair, but there is an aftermarket repair kit from HeaterTreater that will address and solve the problem. Complete diagnostic information and diagrams are available.