Boiler
F10
The F10 error code on a Viessmann boiler indicates a flow temperature sensor fault — a safety-critical issue that will typically lock out your boiler and leave you without heating or hot water. It needs prompt attention, but there are one or two checks you can safely do yourself first.
What does this error mean?
The F10 code means your Viessmann boiler’s flow temperature sensor (also called the NTC flow sensor) is sending a reading that falls outside the expected range — either far too high, too low, or no signal at all. The boiler’s control board monitors this sensor continuously to ensure the water it’s heating stays within safe limits. When the signal is missing or implausible, the boiler shuts down as a precaution to prevent overheating or damage to internal components.
Est. repair cost
£150–£300 for a Gas Safe engineer to diagnose and replace the flow temperature sensor (parts and labour)
🔍 Likely causes
🛠️ Step-by-step fix
Check your boiler’s pressure gauge — it should read between 1 and 1.5 bar when cold. If it’s below 1 bar, repressurise the system using the filling loop (your boiler manual has model-specific instructions). Low pressure alone can trigger sensor errors.
Press the reset button on your Viessmann boiler (usually a flame symbol with a line through it). Hold for 3 seconds and release, then wait 5 minutes to see if the boiler fires up normally. A one-off sensor glitch can sometimes clear this way.
Check your boiler display again after the reset. If F10 returns within a few minutes, the fault is persistent — do not keep resetting repeatedly, as this can mask a worsening issue.
Bleed any radiators in your home to remove trapped air from the system, then re-check boiler pressure and top up if needed. This addresses one of the simpler underlying causes.
If the error persists after the above steps, stop here. Do not attempt to access internal boiler components, disconnect sensors, or inspect wiring yourself — this is a gas appliance and all internal work must be carried out by a Gas Safe registered engineer.
📞 When to call a professional
If the F10 error returns after a reset and basic pressure/bleeding checks, call a Gas Safe registered engineer — do not attempt to inspect or replace the flow sensor yourself. Tell the engineer: ‘My Viessmann boiler is showing fault code F10, which I understand relates to the flow temperature sensor.’ This helps them arrive prepared with the right diagnostic tools and a likely replacement part, saving time and call-out cost.
⚖️ Repair or replace?
Replacing a Viessmann NTC flow temperature sensor typically costs £150–£300 including parts and labour in the UK, depending on your model and location. If your boiler is under 10 years old, repair is almost always the right call. If it’s over 12–15 years old and this isn’t the first fault, consider that a new A-rated Viessmann boiler costs £2,000–£3,500 installed — a repair bill over £400 on an ageing unit rarely makes financial sense.