Boiler
F12
The F12 error code on a Viessmann boiler indicates a flow temperature sensor fault — specifically that the sensor is reading an implausibly high temperature or has failed entirely. It’s a moderately serious fault that will lock your boiler out until it’s resolved.
What does this error mean?
Your Viessmann boiler constantly monitors the water temperature inside the heat exchanger using a flow temperature sensor (also called a flow NTC sensor). When the F12 code appears, the boiler’s control board has detected that this sensor is returning a reading outside its expected range — either far too high, erratic, or no signal at all. As a safety measure, the boiler shuts down to prevent overheating damage.
Est. repair cost
£150–£300 for flow sensor replacement; £300–£600 if PCB replacement is needed
🔍 Likely causes
🛠️ Step-by-step fix
Perform a manual reset: press and hold the reset button on your Viessmann boiler for 3–5 seconds. If the boiler fires up and runs normally, monitor it closely for 24 hours to see if F12 returns.
Check your system pressure: look at the pressure gauge on the boiler front panel. It should read between 1–1.5 bar when cold. Low pressure reduces water flow and can trigger temperature faults. Repressurise via the filling loop if needed (consult your boiler manual for instructions specific to your model).
Bleed your radiators: trapped air in the system reduces circulation. Use a radiator bleed key on each radiator, starting from the ground floor, until water (not air) flows steadily from the bleed valve.
Inspect the boiler casing externally for any visible signs of overheating, scorching, or burning smells. Do not remove the boiler casing yourself — if you notice any of these, stop and call a Gas Safe engineer immediately.
If F12 returns after a reset, do not attempt further DIY. The fault requires a Gas Safe registered engineer to test the flow NTC sensor resistance with a multimeter, inspect wiring looms, and assess the pump and heat exchanger — all of which involve working on gas-connected components.
📞 When to call a professional
Call a Gas Safe registered engineer if the F12 error returns after a single reset, or immediately if you smell gas, see scorching, or hear unusual noises from the boiler. Tell the engineer: ‘My Viessmann boiler is showing an F12 fault code — I believe it’s a flow temperature sensor issue.’ This helps them arrive with the right parts. Do not attempt to open the boiler casing or disconnect any components yourself — this is illegal for unqualified individuals on gas appliances and dangerous.
⚖️ Repair or replace?
A flow NTC sensor replacement by a Gas Safe engineer typically costs £150–£300 including parts and labour — a straightforward job on most Viessmann models. If the PCB is at fault, expect £300–£600. For boilers over 10–12 years old, weigh repair costs against a new Viessmann Vitodens 050-W or 100-W, which starts at around £1,500–£2,200 fully installed; if your boiler is ageing and repairs are stacking up, replacement is the smarter long-term investment.