Mercedes-Benz W203: automatic transmission limp mode and speed sensors
W203: why does the automatic go into limp mode or drop into neutral?
The cause is often the transmission conductor plate, which carries the speed sensor signals. Typical fault codes are P220A (sensor comparison not plausible) and P2500 (impermissible gear ratio). Limp mode usually clears when the engine is restarted.
What should be checked first when fault codes point to the conductor plate?
The seal of the transmission plug may leak, letting oil travel along the wires into the control unit and contaminate it. Check the connectors and the control unit for oil leaks before looking at the internals of the transmission. Also check the oil level and quality.
How is the fault located?
The fault memory is read with a diagnostic tool and cleared, then the car is driven until limp mode returns. The codes are then read again. This shows whether the fault is permanent or intermittent. The conductor plate is often replaced at the same time as the transmission oil is renewed.
What other causes can produce the same symptom?
A slipping transmission goes into limp mode when the slip exceeds the limit; the cause can be worn clutches, the wrong or too little oil, or a pressure leak. A broken shifter unit connector or glycol in the transmission oil can also cause limp mode. Transmission repair belongs in a workshop.