If you are getting no resistance at the pressure switch then it sounds like its stuck open, one way to test to see ifs its the pack is to remove the to wires on the switch and you can use a paper clip and short out the two leads, you will have to do this with the power on. If the flow message goes away then you know its the pressure switch if it does not then it a problem with the pack. The pack knows that it has sent voltage to the pump and if the pack does not see the pressure switch closed(shorted) then it will trip the FLO message.