I had the same problem - it's a bad resistor. Most mecahnics can change this and it didn't cost a lot if I remember.
Could also be a short to ground. If it runs all the time that's what it probably is. Get a wiring diagram and figure it out
The blower motor resistor pack is defective. Replace it.
Replace heater motor resistor.
Why does your 2001 Nissan Pathfinder heater fan stay on high even after I try to shut it off?"
your tranmission is shot.
Maybe because your climate control system needs to be repaired....
Trouble code P0599 means: Thermostat Heater Control Circuit High
12 volts on the low end, 18 volts on the high end.
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If the engine is at the proper temperature and the coolant is full, you have a restricted heater core.
It is the resistor, replace it.
its probobaly broken get it a service