I don't understand how they are keyed between the sensor and the indicator? The Green Sensor-Right Front appears to have failed. There are two parts to this unit--the Valve Stem assembly and the Black Sensor Unit that houses the electonics. Do I have to buy a complete "Green Sensor Unit"? Or since I have a spare new Sensor Electronics unit, can I just take the valve stem out of the failed unit and intall it in the new sensor?

Bottom line I guess, is do you need a special tool to re-sync the sensors to the control indicator?