An auxiliary system with purge control automatically supplies diesel exhaust fluid (DEF) to an onboard DEF tank of a diesel engine to enable prolonged unattended operation. The system includes an auxiliary DEF tank, an auxiliary DEF supply line, a controller, a pump, an air inlet, and a three-way valve configured to switch the pump inlet between the auxiliary DEF tank and air. In response to low-level DEF, the pump delivers DEF through the supply line to replenish the onboard DEF tank. The controller may automatically calculate onboard DEF tank volume based on the delivered volume of DEF, and DEF level data received from an ECM, to enable replenishment control regardless of engine make and model. In response to high-level DEF, engine stoppage, or other system fault, the controller switches the valve to air and runs the pump for a predetermined time to purge DEF from the supply line.