I found this: 641 O Shift Solenoid 3 (SS3) Circuit Failure.
DTC 641
Shift Solenoid
3 circuit failure.
This is a shift solenoid within the Transmission on the valve body. you might be able to get at it once the pan is dropped. Solenoids in transmissions usually aren't too hard to change, just remember how the wires are routed or take a pic.
Have you reset the code and driven it to see if it comes back? How does the vehicle react when the code is set?
found more:
There is an easy and inexpensive way to check your solenoids to see if they are bad. Get out your ohm meter and check the resistance of the solenoid. Hook the wires from your ohm meter to the two leads coming off the solenoid. If you get no reading that means the solenoid has failed due to a break in the internal coil wiring. If you get a very low reading that meaning there is short in the internal coil. Most properly functioning solenoids will return a reading of somewhere between 20 – 30 ohms. Solenoids are fairly inexpensive as far as replacement parts go and are also fairly easy to replace.