Or in the interim, round up a cat tech, take the truck to them and have them scan it and analyze what the code is when the switch is engaged. The cost of one session however could equal the cost of the laptop and interface to start doing it yourself…
The ECU provides the power to control the fan solenoid and aux cooler relay. When it wants fan off, it powers the solenoid with 24v to provide the air that releases the fan clutch. When it wants fan on, it removes the 24 V to release the air and engage the fan. This power from the ECU passes thru the fan switch. When you flip the switch to disable the fan, it shifts the fan control power from the ECU to another 24V source which keeps the fan disabled for as long as the switch is in the fording/disable position.
could be the switch is mis-wired or has an issue, and when you set it to fan disable it is overloading the power output from the ECU? I have seen 2 different pinouts listed for the dash switches, so if someone at some point replaced a switch and used the wrong part, its config might not match the appropriate wires in the switch plug.
Do you have Air conditioning? I recall the air-con install taps something associated with that switch… I recall helping someone with a issue associated with that A/C wiring once upon a long time ago, but I do not recall the details…