Transmission doesn't shift or downshifts
The solenoids are in valve body are most likely stuck, this could be do to lack of service the transmission fluid needs to be replaced every 30k miles to avoid expensive repairs.
The valve body most likely needs to be replaced, if it has not already cause the transmission to go bad by grinding or slamming gears.
you can try to replace the transmission fluid which has a slim chance of working but might be worth a try since the valve body is over $900, install a used transmission if also an option is the transmission turns out to be bad.
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