The Nintendo Switch’s cooling system is complex, sharing resources with the Joy-Cons that power the Joy-Cons rails.
Overheating Joy-Con Issues
If the console displays an overheat warning signal and eventually shuts down, or if you encounter problems charging the Joy-Cons while connected to the system, it’s likely a problem with the fan circuitry.
Diagnostic Steps
No fan spinning and/or Joy-Con charging is often due to a small MOSFET located above the M92T36.
Sometimes this MOSFET disconnects, meaning the 3V voltage can’t power the circuitry. Sometimes the MOSFET is perfectly fine; the problem could be a faulty fan, a faulty Joy-Con rail, or corrosion around the wiring.
Repair Steps
The simplest solution is to briefly inject 3V into the fan circuitry. Just tap the 3V wire two or three times for a second or two, and the MOSFET will usually magically recover.
I personally know of this method working well over the past 12 months without any warranty returns.
Make sure no Joy-Cons are connected, but ensure the console is powered on during this process. Sometimes leaving the computer on will immediately start the fan, because the CPU temperature is high enough that it needs it, which proves the method is successful. If the voltage injection method fails, replacing the MOSFET is the only option.
