@Chrisr - if two different controllers are having issues, then the thing that is in common is the pump start relay. Check the wiring to make sure there isn’t a short on the pump relay wiring. If there is still the error, then disconnect the P/MV wire to see if the system will run (albeit with reduced pressure) and not throw the error on the P/MV circuit. One last test would be to swap the P/MV wire and a zone wire (pick one), then run that zone and see if there is an error and if the error followed the P/MV wire.
