Boiler
F05
The F05 error code on a Viessmann boiler indicates a flue gas temperature sensor fault — specifically that the sensor is detecting abnormally high exhaust temperatures or has failed entirely. This is a safety lockout, meaning your boiler has shut itself down and won’t restart until the underlying issue is resolved.
What does this error mean?
Your Viessmann boiler continuously monitors the temperature of the flue gases leaving the heat exchanger. When it logs F05, it has detected that those gases are either dangerously hot — suggesting poor combustion or a blocked flue — or that the sensor itself has developed a fault and is sending incorrect readings. Either way, the boiler locks out as a precaution to prevent damage or a potential safety hazard. This is not an error you can simply ignore or reset your way past without identifying the root cause.
Est. repair cost
£150–£350 for flue gas sensor replacement by a Gas Safe engineer; new boiler installation £1,500–£2,500+
🔍 Likely causes
🛠️ Step-by-step fix
Check the flue terminal outside your home — ensure it is not blocked by debris, birds’ nests, snow, or any obstruction. Clear any visible blockage carefully from the outside only.
Press the reset button on your boiler (usually marked with a flame symbol or a circular arrow) and hold for 3 seconds. Allow the boiler to attempt a restart and observe whether F05 returns immediately.
If the error clears but returns within minutes or hours, do not keep resetting — repeated lockouts indicate a persistent fault that needs diagnosis, not masking.
Check your boiler’s pressure gauge is between 1–1.5 bar when cold. Low pressure can contribute to poor circulation and overheating, though it will not directly clear F05 on its own.
Do not attempt to inspect, test, or replace the flue gas sensor, heat exchanger, or any internal gas components yourself — this requires a Gas Safe registered engineer by law in the UK.
Contact a Gas Safe registered engineer and quote the F05 code. Ask them to check the flue gas sensor resistance, flue integrity, and heat exchanger condition.
📞 When to call a professional
Call a Gas Safe registered engineer immediately if the boiler will not reset, if F05 returns within a short time of resetting, or if you notice any smell of gas, unusual smells from the flue, or soot marks around the boiler casing. Tell the engineer you have a Viessmann boiler showing fault code F05 and that you suspect a flue gas temperature sensor or flue integrity issue — this helps them bring the right parts. Do not attempt to bypass the lockout or tamper with the sensor or flue internally.
⚖️ Repair or replace?
If your Viessmann boiler is under 10 years old and in otherwise good condition, repair is almost always the right call. Replacing a flue gas NTC sensor typically costs £150–£350 all-in with a Gas Safe engineer, which is excellent value compared to a new boiler. If your boiler is over 12–15 years old and this fault is accompanied by other recurring issues, it may be worth getting a replacement quote — a new A-rated Viessmann Vitodens 050-W or 100-W starts at around £1,500–£2,500 installed, with long-term energy savings that can offset the cost within a few years.