If your vehicle is showing transmission temperature warning light is on, here are the most likely causes and the parts you'll need to fix each one.
Low fluid levels due to a leak, or old, degraded fluid that has lost its thermal properties, prevents the transmission from properly cooling itself. This is frequently caused by underfilling during a recent fluid change.
Towing heavy loads, driving in deep snow/sand, or navigating steep grades puts immense strain on the transmission, generating excess heat that the factory cooling system cannot handle.
The sensor responsible for monitoring the fluid temperature can short out or fail, sending a false overheating signal to the dashboard even when the transmission is running at a normal temperature.
A heavily restricted or clogged internal transmission filter prevents fluid from circulating properly to the cooler, causing the fluid trapped inside the transmission to overheat rapidly.
The transmission cooler (often integrated into the vehicle's main radiator) can become clogged with debris or fail internally, completely halting the heat exchange process.
Worn internal clutch packs or a failing torque converter will slip instead of locking up. This constant friction generates massive amounts of heat that no cooler can overcome.