Most likely a failed blower resistor.
TPS volts high. You need to test the throttle postion sensor.
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.
Downstream O2 sensor circuit volts high.Downstream O2 sensor circuit volts high.
Replace heater motor resistor.
Why does your 2001 Nissan Pathfinder heater fan stay on high even after I try to shut it off?"
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
Maybe because your climate control system needs to be repaired....
Trouble code P0599 means: Thermostat Heater Control Circuit High
Your heater fan only working on high is probably the little coil sensor heater assembly located under the hood, it will be on the far left side of vehicle if your standing in front of it on the side of the air plenum. There is two screws holding it into the plastic chamber and a wire harness which you must unplug. Its right next to the transmission dipstick handle which is probably yellow like mine. I replaced mine for around 30.00 from the dodge dealer and that fixed it. Good luck, hope this helps you!
If the engine is at the proper temperature and the coolant is full, you have a restricted heater core.
P0406 Exhaust Gas Recirculation Sensor A Circuit High