Boiler
F07
The F07 error code on a Viessmann boiler indicates a fault with the flue gas temperature sensor, and while it won’t always leave you without heating immediately, it’s a fault that needs prompt attention to keep your boiler running safely and efficiently.
What does this error mean?
The F07 code tells you that your boiler’s flue gas temperature sensor has detected an abnormal reading — either too high, too low, or an implausible signal that doesn’t match the boiler’s operating conditions. Viessmann boilers use this sensor to monitor the temperature of exhaust gases leaving through the flue; if readings fall outside safe parameters, the boiler locks out to prevent damage or a dangerous situation. In plain terms, the boiler thinks something is wrong with how hot the outgoing gases are, and it’s refusing to fire until that’s resolved.
Est. repair cost
£150–£350 for sensor replacement by a Gas Safe engineer; no cost-effective DIY option for this repair
🔍 Likely causes
🛠️ Step-by-step fix
Step 1 — Reset the boiler: Press and hold the reset button on your Viessmann boiler for 3–5 seconds. Many F07 faults caused by a temporary glitch will clear immediately. If the boiler fires up and runs normally for a full heating cycle without the code returning, monitor it for 24 hours.
Step 2 — Check your flue outlet externally: Go outside and visually inspect the flue terminal (the pipe or grille on your external wall or roof). Make sure it isn’t blocked by debris, a bird’s nest, ice, or overgrown vegetation. Do not attempt to access the flue internally.
Step 3 — Check for recent power issues: If your home recently had a power cut or surge, the fault may be a false trigger. Reset once more and observe. If the code returns within an hour of firing, stop here.
Step 4 — Do not attempt further DIY: The flue gas temperature sensor is an internal gas appliance component. Replacing it, testing it with a multimeter, or inspecting internal flue connections requires working on a live gas appliance — this is illegal for unqualified individuals under UK Gas Safety (Installation and Use) Regulations 1998. Call a Gas Safe registered engineer.
Step 5 — Note the pattern: Before calling an engineer, make a note of when the fault appears (cold start, mid-cycle, after long runs), how often it resets successfully, and whether you’ve noticed any unusual smells or sounds. This information helps the engineer diagnose the fault faster.
📞 When to call a professional
Call a Gas Safe registered engineer if the F07 code returns after a reset, if the boiler won’t reset at all, or if you notice any smell of gas, unusual noises, or visible soot around the boiler or flue. Tell the engineer: ‘My Viessmann boiler is showing an F07 fault code — I believe it’s related to the flue gas temperature sensor.’ This gives them a clear starting point and ensures they bring the right parts.
⚖️ Repair or replace?
A flue gas temperature sensor replacement on a Viessmann boiler typically costs £150–£350 including parts and labour, depending on your boiler model and engineer’s rates. If your boiler is under 10 years old, repair is almost always the right call — Viessmann boilers are well-built and worth maintaining. If the boiler is over 15 years old and this is one of several recent faults, get a quote for a new boiler alongside the repair cost; a new A-rated condensing boiler starts around £1,800–£2,500 installed.