A system for purging a device for injecting fuel into an exhaust system of an engine comprising a device having an input and an output coupled to an exhaust system, at least one air valve and at least one fuel valve. The air valve has an input an input coupled to an air supply and an output coupled to the device through a check valve. The fuel valve has an input coupled to a fuel supply and an output coupled to the device through a check valve. When the air valve is open and the fuel valve is closed, air flows from the air supply to the device and is injected into the exhaust system of the engine. When the air valve is closed and the fuel valve is open, fuel flows from the fuel supply to the device and is injected into the exhaust system of the engine.