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Hi I am Daniel
You have 4 speed gears are you sure its not 4th gear out?
Transmission General Description
The new automatic transmission AISIN (81-40LE) is an electronically controlled 4-speed automatic transmission with lock-up mechanism. The transmission is mainly composed of the torque converter with lock up clutch, newly developed 4-speed planetary gear unit, the hydraulic control system and the electric control system.
Hydraulic Control System Based on the hydraulic pressure created by the oil pump, the TCM sends signals to the solenoid and the hydraulic control system governs the hydraulic pressure acting on the torque converter, planetary gear, clutches, and brakes in accordance with the vehicle driving conditions.
you can flush the system in case there is debree. and make sure fluid is up to level.
When you lose 4th gear it can be the pnp switch or the shift solenoid 1 circ uit or 2 circuit. The TCM transmission control module. The throttle opening signal or the valve body. and of course transaxle internal parts.
So testing is best and and yuou will need to get a scanner on it and do a functions test to accuratly find the issue. If you find clutch debree in the fluid it will need a rebuild or if fluid smells burnt. But alot of times it can be fixed without even taking transmission out of car.
"When you lose 4th gear it can be the pnp switch or the shift solenoid 1 circuit or 2 circuit. The TCM transmission control module. The throttle opening signal or the valve body. and of course transaxle internal parts." is your answer But I wanted to give you more education for a better understanding.