What does it mean?
The transmission control system has measured a fluid temperature above its allowed limit for longer than the calibration permits. Automatic transmissions rely on the fluid for lubrication, hydraulic pressure and clutch cooling, and its properties change quickly once it gets too hot, so the module protects the gearbox by locking out certain gears, holding the torque converter locked or reducing engine torque. The temperature signal usually comes from a sensor inside the valve body or the pan. The important question is whether the fluid really was that hot or whether the reading is wrong, and that is answered by comparing live data with a direct measurement of the pan. Repeated overtemperature events shorten transmission life considerably.
Symptoms
- Transmission warning message or a gearbox temperature light on the display
- Harsh, delayed or flared shifts, and sometimes a refusal to shift into the highest gear
- Torque converter lockup dropping out, giving higher engine speed at cruise
- A burnt smell from under the vehicle after heavy use
- Reduced engine power once the module starts protecting the transmission
- Shuddering at light throttle when the fluid has degraded
Likely causes
- Low, old or badly degraded transmission fluid that can no longer carry heat away
- Restricted or blocked transmission fluid cooler, or kinked and partly collapsed cooler lines
- Debris blocking the airflow through the cooler at the front of the vehicle
- Heavy operation such as towing, long climbs or repeated stop and go traffic beyond the design duty
- Slipping clutches or a torque converter clutch that will not lock, both of which generate large amounts of heat
- Stuck thermostatic bypass valve keeping the fluid away from the cooler
- Failing cooler pump or auxiliary cooling fan where one is fitted
- Temperature sensor reading high because of a wiring or sensor fault, which produces the code without real overheating
How to fix it
- Compare the live fluid temperature with an infrared measurement of the transmission pan; a large difference means the sensor or its wiring is the problem
- Check fluid level and condition using the correct procedure and temperature window for that gearbox, and look at colour and smell
- Inspect the cooler and its lines for restrictions, kinks and external blockage, and check flow through the cooler
- Verify the operation of the thermostatic bypass valve if the vehicle has one
- Log torque converter slip and lockup behaviour on a road test; a converter clutch that will not hold is a common heat source
- Confirm any auxiliary cooler fan or pump runs when commanded
- Replace the fluid and filter with the specified type once the cause is understood, then repeat the road test while logging temperature