Boiler
F06
The F06 error code on a Viessmann boiler indicates a fault with the flow temperature sensor (NTC sensor), and while it won’t always leave you without heating immediately, it will trigger a lockout if left unresolved — so it needs prompt attention.
What does this error mean?
Viessmann boilers use an NTC (negative temperature coefficient) sensor to monitor the temperature of the water flowing out of the boiler. When the boiler’s control board receives a reading from this sensor that is outside the expected range — either implausibly high, low, or no signal at all — it registers an F06 fault and may shut the boiler down as a safety precaution. In plain terms, the boiler can’t confirm the water temperature is safe, so it stops operating until the issue is resolved.
Est. repair cost
£150–£300 for NTC sensor replacement; £300–£600 if the PCB is at fault (parts + Gas Safe labour)
🔍 Likely causes
🛠️ Step-by-step fix
Check the boiler pressure gauge first — if the pressure is below 1 bar, top up via the filling loop to 1–1.5 bar, then press the reset button and observe whether F06 clears. Low pressure can cause temperature anomalies.
Press and hold the reset button (usually marked with a flame symbol or ‘Reset’) for 3–5 seconds. Some transient sensor faults clear after a single reset, especially after a power fluctuation.
Switch the boiler off at the mains, wait 30 seconds, and switch it back on. A full power cycle can clear false fault readings on the control board.
Check the condensate pipe (the white plastic pipe usually running to an outside drain or internal soil stack) is not frozen or blocked — a blocked condensate can cause knock-on faults. Pour warm (not boiling) water over a frozen external section.
Do NOT attempt to access, test, or replace the NTC sensor or any internal boiler components yourself. Viessmann boilers are gas appliances — internal work must only be carried out by a Gas Safe registered engineer. Stop here and book a professional.
📞 When to call a professional
If the F06 fault returns after a reset and pressure top-up, stop attempting DIY fixes and contact a Gas Safe registered engineer. Tell them: ‘My Viessmann boiler is showing an F06 fault code — I believe it relates to the flow NTC temperature sensor.’ This gives them a head start and ensures they bring the right parts. Do not attempt to open the boiler casing or interfere with the sensor wiring yourself — this is illegal for unqualified individuals on a gas appliance and could be dangerous.
⚖️ Repair or replace?
An NTC sensor replacement on a Viessmann boiler typically costs £150–£300 all-in with a Gas Safe engineer, including parts and labour. If the fault lies with the PCB, costs rise to £300–£600, which on a boiler over 10 years old makes replacement worth considering — a new Viessmann Vitodens 050-W starts around £1,500 installed. For boilers under 8 years old in otherwise good condition, repair is nearly always the right call.