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Do you have experience using a digital ohm multi meter?
Can the blower speed be adjusted?
Is this system manual temperature control, or automatic?
There is a 4 wire connector at the blower fan, measurements will need to be taken here to properly diagnose the concern.
I was asking if the blower fan speed changes when you manually adjust the speed with the controls.
The most likely failure is a blower fan speed module, aka blower motor power stage.
Check for voltage with the meter on direct current.
1. On the 4-wire connector at the blower fan, check the Blue wire for voltage. There will be a small voltage sent on that wire to activate the fan. If voltage goes away when the key is off the power stage is bad.2. One could also remove that terminal from the connector. If the fan stays running, the power stage is bad.3. If there is voltage on that wire or if the blower motor stops running when the wire is removed, fault trace the Climate Control Module (CCM). The CCM gets key on information from the Controller Area Network (CAN), but gets powered whenever the door is opened.
Tip:The fan power stage is located inside the blower motor assembly. Remove the blower fan to gain access.