Whenever tha heater/AC is on in any mode, the fan will run on low speed
FUSE!
It could be the heater core leaking.
Low coolant? Bad thermostat? Heater core airbound? Heater core restricted or plugged? Defective radiator cap not building sufficient pressure?
Probably need to add antifreeze. Check the Radiator not the overflow tank. If you are low on antifreeze it will not circulate through the heater core.
Bad/sticky thermostat.Cooling fan(s) malfunction.Bad head gasket.Cracked head.
No...
Change your heat control sensor it mount on your heater motor on most models, it reduces the amount of voltage to the fan motor.
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A leaking heater core will cause the engine to overheat because the coolant is leaking out.
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Did you check your thermostat. another reason could be your heater core is plugged.
Possibly a bad heater core. Could be simple stuff though. Have you checked your coolant level? Sometimes those "doors" in the dashboard for the heat and a/c are not opening and closing properly.