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Start by checking the electrical connectors on the top of the transmission for any corrosion.
The next step- you can ohm out the resistance of the solenoids in the valve body- without the factory pinout harness you would have to drop the tranny pan. If it is not the solenoid itself, it could be a fault/open in the internal wiring.
The solenoids are arranged around the perimeter of the "valve body", a large metal block that routes the tranny fluid pressure within the transmission. After dropping the tranny pan, you can carefully disconnect the wiring to the solenoids, and ohm out each of the solenoids. Two of them have the same resistance value and the other five all have the same (different from the first two) resistance value. The matching solenoids have the same color body.
If it is not the solenoid, then it could possibly be the wiring harness that runs from outside the transmission to the solenoids.
At the dealership, we would almost always replace the entire valve body. You may be able to source the solenoids separately.
You would need to clear the code after any repair, otherwise the computer may still think the fault is there and react accordingly even if there is no actual problem. The tranny control module would not need to be reprogrammed in any way
No matter what, if it turns out to be a solenoid, you will be replacing the transmission pan gasket and fluid, which runs around $100 for parts and up to two hours labor ($103/hr here) To replace just a solenoid I would think would not be more than a hundred dollars in parts, but the whole valve body can be expensive depending on application- $500-$800 are average costs, and that would probably add an hour (solenoid) or two (valve body) to the job. (just an estimate, may vary depending on shop) Depending on mileage, some shops may not like to do just a solenoid or just a valve body, if the transmission has 150k on it for example they may just recommend a whole new transmission, as draining fluids can cause suspended particles to clog orifices.