Viessmann [F.471] fault codewhat it means and how to fix it

F.471 is the row for the pressure sensor itself, as distinct from what it measures: "No heat demand — System pressure sensor not available, lead break or short circuit — Check system pressure sensor (plug aND). Check lead and plug-in connection. Check whether the supply voltage to the sensor is 5 V." "No heat demand" is Viessmann's way of saying the control unit will not accept a call for heat while it cannot read pressure; a boiler that could not tell whether it was full of water would be a boiler that could boil dry, so the safe answer is to do nothing until the reading is back. Neither the burner nor the pump runs, and the code will not respond to a reset while the sensor is still unreadable.

The sensor is a small electronic transducer on the hydraulic block, fed at 5 V from the HBMU heat management unit through the plug Viessmann label aND. Its three failure modes are in the row: not available (plug off or sensor dead), lead break (a chafed or unclipped cable, the reading intermittent as the cable is disturbed) and short circuit (water in the connector after a leak above it is the classic). The contractor checks supply voltage first, because a dead 5 V rail points to the HBMU rather than the sensor. There is nothing in the operating instructions for the householder here beyond noting the code, and F.471 is not a pressure problem you can top up your way out of — if the pressure-gauge symbol on the display shows a reading at all, it cannot be trusted while this code is present.