Vaillant · Current Vaillant F-code boilers
F.20Safety thermal cut-out operated
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.