Auxillary Fuel Tank

Your aux tank also needs a vented cap, just as the main tank does. Also,gravity or siphon thru your transfer pump may be keeping the truck tank full or possibly level in the man tank is falling slower than the computer thinks it should which can throw a code. Mine would do this, with diesel, until I put a solenoid in line with the pump that opens when I turn the pump on, closes pump off. That stopped the fuel from flowing to the main tank all the time. Now fuel level goes up and down like the computer thinks it should.