The transistor that drives the various levels of the blower motor is shot. It is shorted internally. This part is around $100 at your dealer. unless you had a bad relay and some used car dealer just jumped the relay with a wire. not saying this ever happened to me though...
Common causes, blower motor resistor, blower motor, wiring or controls
Recommend you check the blower motor resistor.
Possibly a defective Blower Motor Resistor Pack.
My 2005 does the same thing, just found out it's the blower motor control module. Hope that helps out!!
Why does your 2001 Nissan Pathfinder heater fan stay on high even after I try to shut it off?"
it needs a new blower motor resistor ....
most likely a defective blower module under the hood
Blower fan relay is stuck or fused contacts, replace it
The blower motor resistor block is located on the underside of the HVAC housing (Heater Box) next to the blower motor. The blower motor is located at the right side of the dashboard. You can access the blower motor resistor block from under the dashboard. A Word of Caution! Stay clear of the blower motor resistor block heat sink - It could be Hot. Incidentally, I have read about people working on their dashboards and setting off the airbag. It is a good thing to disconnect the negative battery cable from the battery and make sure that it can not touch the battery post. Wait at least a couple of minutes for the airbag system to discharge.
THE BLOWER MOTOR RESISTIOR IS FRIED. CHEAP CHINESE PART. WHEN IT BLOWS THE DEFAULTS IS FOR THE BLOWER MOTOR TO BE STOCK ON FULL-BETTER THAN NOT WORKING AT ALL. THE PART IS CHEAP$20-$25.00. THE INSTALL IS VERY HARD. LET THE DEALER DO IT.
Bad head unit. If your fuse number 17 keeps blowin, that is why also.
The blower control module has gone bad. Carquest has a new one for $20-$25 dollars.