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.84 means the two pipe sensors disagree in a way the software will not accept — classically swapped flow and return NTCs, or one or both not fitted on the pipes.
Vaillant meaning: Flow/return NTC difference implausible
Possible causes
- Flow and return NTCs inverted
- One or both NTCs not fitted correctly
- One defective NTC reading nonsense
Possible solutions
- Check physically: flow NTC on the flow pipe, return NTC on the return pipe. Swap them if they were crossed (also a cause of F.23).
- Both clips must be tight on bare metal.
- Measure both sensors; replace a failed NTC.
Possible parts needed
- Flow/return NTC 193592 or 0020068120 (replace as a pair if ages differ a lot)
