Hi, shift solenoid is located under the transmission pan. Shift solenoid to verify is bad you will need a multimeter capabile of testing ohms. Remove the pan, and connectors at all one wire solenoids and two. Probe the connection of where the wire plugs into on the solenoid, and with the other lead, touch the valve body, if a one wire solenoid. If a 2 wire solenoid, probe each connection of the solenoid with one on each probe of multimeter. If you see OL, or thousands of OHMS, that solenoid is at fault. When testing all solenoids, if you find none of them are giving a reading of OL, or thousands of ohms, the solenoid is not at fault, and need to further test, such as seeing if transmission fluid seems burned (internal fault in transmission will require removal and repair, or replacement). An internal fault in a transmission usually will give a shift solenoid performance code, tricking the person repairing into replacing a solenoid, while it may fix nothing. OHM check all one wire solenoids as described above, and if okay, and fluid smells burned, time to remove transmission disasemble and repair or replace as needed.
When removing the pan, use a large drain pan, will cause a mess!