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.83: at burner start the flow and/or return NTC did not see a temperature rise (or the rise was too small). Usually not enough water, air, or an NTC not actually on the pipe — not a random electronics code.
Vaillant meaning: Flow and/or return NTC temperature-change fault
Possible causes
- Low water / air in the boiler
- Flow or return NTC not on the correct pipe or sitting off the pipe
- Pump not circulating at ignition
- Blocked heat exchanger so the sensors do not see heat
Possible solutions
- Check cold pressure (about 1.0–1.5 bar) and bleed the boiler / high points. See boiler pressure and F.22.
- Confirm both NTCs are tight on flow and return.
- Prove the pump runs when a heat demand starts (F.23 / F.24 / F.75 overlap).
- Replace an NTC only if mounting is correct and that sensor never moves in diagnostics (D.40 / D.41).
Possible parts needed
- Flow/return NTC 193592 or 0020068120
- Pump
- Pressure sensor if pressure is unstable (0020059717)
- System water / inhibitor / filter after sludge
