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.85 is more specific than F.84: both sensors are on the same pipe, or on the wrong pipes entirely.
Vaillant meaning: Flow and return NTCs incorrectly fitted
Possible causes
- Both NTCs clipped to the same pipe
- Both fitted to the wrong pipes after a heat-exchanger or pump change
Possible solutions
- Refit: one NTC on flow, one on return, as labelled in the manual.
- Reset and run a demand. Flow should rise first.
Possible parts needed
- Usually no new parts — refit the existing NTCs
- NTC 193592 or 0020068120 if a clip or sensor was broken
