Theory Of Operation

Each of the Diesel Exhaust Fluid (DEF) system hoses has a built in heater to warm the fluid in the lines. The three line heaters share a common power supply via the DEF Line Heater Relay which is controlled by the DEF Dosing Control Unit. The DEF Dosing Control Unit controls the heaters individually through the line heater return circuits. The three line heater circuits are monitored at the return circuit. The DEF Dosing Unit Module will illuminate the MIL light via the PCM immediately after the diagnostics runs and fails and the line heaters will be disabled. The DEF Dosing Control Unit will turn off the MIL light via the PCM immediately after the monitor runs and passes.