If your vehicle is showing service transmission message on dashboard, here are the most likely causes and the parts you'll need to fix each one.
Low fluid levels from a leak or old, degraded fluid can cause slipping and trigger the warning message. A recent improper fluid change (wrong fluid or under-filled) can also cause this issue.
Modern electronic transmissions are highly sensitive to voltage drops. A weak battery can trigger false transmission faults and warning messages during engine startup.
Shift solenoids direct fluid to change gears. If one fails electrically or gets clogged with debris, the transmission will shift harshly or get stuck in gear, triggering a service message.
The computer relies on input and output speed sensors to know when to shift. If a sensor fails, the transmission may go into 'limp mode' and display the warning.
If the sensor incorrectly reads that the transmission is overheating, it will trigger the service message to protect the vehicle from perceived damage.
The computer that controls the transmission can fail due to heat, vibration, or electrical shorts, causing erratic shifting and warning messages.
Severe internal wear of clutches, bands, or the torque converter will cause slipping and metal debris, requiring a major overhaul or replacement.