Heater wont turn off - 1998 Plymouth Voyager

You may just need to replace the temperature control unit. You can find used ones online for pretty cheap.

in the under hood fuse block pull the fuse labeled "blower" or the relay with the same label.

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that fuse powers pcm fuel heater and some other things. when the fuse blows the pcm shuts down. causing it to not run. usually its the heater that is bad. it is located at the back driver side by the fuel pump. you can unplug it and see if it starts. if so you will need to replace the heater

could be the blower motor

It could be your blower switch, but more than likely it's a resistor that is usually a part of the blower assembly, and it operates the speeds. Doesn't cost much but can be tricky installing. If you had one or two speeds not working, initially, eventually all may stop. If fan stopped working and all speeds had been operating, then you may have a bad wire or blower motor. Also, check your manual for fuses, as there may be a compartment inside the vehicle as well as one under the hood.

If you turn the key off and it wont shut off the heater then there are some interesting things you need to check. The first would be the whole ignition switch. When you shut off the key, the power should stop to that accessory. If that is okay then you need to check the controls for the heater. The issue with no heat is one of a blend door being stuck and this could be the controls again.

the most common cause of intermittent no start issues as the crankshaft position sensor on this car.
I need clarification on your heater issue.

Replace the fan control module.

Could be fuse in the motor compartment or resistor relay under the glove compartment or blow motor faulty ground...