Theory Of Operation

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The Diesel Exhaust Fluid (DEF) Heater Control Unit (HCU) operates three heater elements to help eliminate freezing of the DEF. One is the DEF Tank Heater Element (sometimes called the 'Suction Line Heater' or 'Heater 1'). One is the Pressure Line Heater Element ('Heater 2'). The other is in the DEF tank. The HCU interfaces with the Powertrain Control Module (PCM) over two dedicated CAN Bus circuits. The DEF Level/Temperature Sensor reports the temperature of the DEF inside the tank back to the PCM. The PCM determines the heater actuation times and conductance. The HCU receives these commands from the PCM and activates the heater elements as needed. The HCU receives power directly from the vehicle battery to supply the heaters. The PCM illuminates the MIL lamp immediately after the diagnostic runs and fails.