Vaillant · Current Vaillant F-code boilers
F.84Implausible flow/return sensor temperature difference
This code applies to ecoTEC plus, ecoTEC plus remastered, ecoTEC pro, ecoTEC exclusive, ecoFIT pure. Fault codes narrow down the likely problem, but they do not replace a model-specific diagnosis.
What you can safely check
- Check the system pressure and make sure accessible radiator valves are open.
- Allow an overheated boiler to cool and reset once only if the user manual permits it.
- If the fault returns, stop resetting and arrange an engineer.
How an engineer may fix it
Diagnose water circulation, air locks, pump operation, valves, bypass, blockages, heat exchanger and temperature sensors; restore correct flow and test the safety controls.
Vaillant states that meanings can vary by appliance. Confirm the exact model manual before acting.
Manufacturer source: Vaillant UK — Boiler fault codes list. Checked 15 August 2026. The appliance manual and an engineer’s on-site diagnosis take precedence. Never remove the boiler case yourself.