The Reducer Level Sensor (RTS) is used on diesel exhaust vehicles, and its function is to communicate electronically to the Engine Control Module (ECM) the fluid levels in the DEF (Diesel Exhaust Fluid) storage tank. To make this reading, this sensor uses 3 stainless steel pins, in which the length between each one indicates the different levels in the tank. Also, there is a fourth pin, which is used as a ground pin.
When the Diesel Exhaust Fluid remains in the storage tank below the DEF levels set by the vehicle manufacturer, the Engine Control Module will determine that it is too low, which will cause the P203F OBDII fault code to be set.