Transmission Control System Malfunction (MIL Request)
The automatic gearbox has its own control unit (TCM), which decides when to shift using speed, temperature, pressure and selector position sensors. This code doesn't say what the fault is: it only means the gearbox control unit has stored its own code and asked for the engine warning light to come on.
What the car has detected
The gearbox control unit has sent the engine computer a request to switch on the warning light. The specific fault is stored in the gearbox control unit and has to be read there.
It depends on the real fault: the gearbox may work normally or go into limp mode with few gears. Avoid pushing it and read the gearbox codes soon.
Symptoms
- Engine or gearbox warning light on
- Harsh or late shifts
- Limp mode: stuck in one gear
- Often other codes stored in the gearbox
Possible causes
- Any fault stored in the gearbox control unit (sensors, solenoids, pressure, temperature)
- Communication faults between control units (U codes)
- Other engine faults that the gearbox control unit also detects
- Incorrect gearbox oil level or condition
Checks for the garage
- Read the codes stored in the gearbox control unit
- Fix other engine and communication codes first
- Review the freeze frame data from when it appeared
- Check the gearbox oil level and condition if the procedure allows
- Don't replace the gearbox control unit because of this code alone
Next step
Ask for the gearbox control unit's codes to be read (not just the engine's): that is where the real cause is.
Go to a garage to read the gearbox control unit with a full diagnostic tool; a basic OBD reader sometimes only shows P0700.