Gas Safe: Gas work, flue work, and PCB replacement on a gas boiler must be carried out by a Gas Safe registered engineer. Homeowners can check obvious items such as heating pressure, condensate overflow, and whether a control is unplugged — then call an engineer.
Always confirm the part number against the boiler GC number and the installation manual before ordering.
Overview
F.71 means the flow NTC is not changing when it should — stuck reading, poorly clipped to the pipe, or insulated from the pipe. Different from F.00 (open) and F.10 (short).
Vaillant meaning: Flow NTC fault (constant value)
Possible causes
- Flow NTC not clipped / not in the pocket on the flow pipe
- Sensor sitting on paint, insulation, or a dry pocket
- Defective NTC stuck at one resistance
Possible solutions
- Look at the flow pipe: the NTC must be tight on bare pipe or fully in the pocket.
- Run a heat demand and watch D.40 (flow temperature) if you have diagnostic access. A dead-flat reading while the flow pipe is getting hot is F.71.
- Replace the NTC if mounting is correct and the reading still does not move.
Possible parts needed
- Flow NTC 193592 or 0020068120
- Heat-conductive seating / clip if the original is missing
