The engine error P204C Code is necessary thing and if the real meaning of the code is found, then you can try to fix the car engine problem. The general meaning of the code is Reductant Pressure Sensor Circuit Low. This code has a description and it is the reductant compression sensor offers feedback to Powertrain Control Module, which controls system compression by supervisory pump speed using pulse width cadence. The reductant pressure sensor is essential to reductant pump gettogether. Engine Light ON is a symptom and you can follow the symptom to understand the real meaning. A test drive is recommended.