Boiler
E12
The E12 error code on a Viessmann boiler indicates a fault with the flue gas temperature sensor or an overheating condition in the flue system — it’s a safety shutdown that needs prompt attention and should not be ignored.
What does this error mean?
Viessmann boilers use the E12 code to signal that the flue gas temperature sensor has detected readings outside the safe operating range, or the sensor itself has failed. The boiler locks out as a precaution to prevent overheating damage or a potentially unsafe flue condition. Until the underlying cause is resolved, the boiler will not restart normally.
Est. repair cost
£150–£300 for sensor replacement; £400–£600+ if heat exchanger work is required
🔍 Likely causes
🛠️ Step-by-step fix
Step 1 — Reset the boiler: Press and hold the reset button (usually marked with a flame or ‘R’) for 3–5 seconds. If the boiler fires up and runs normally for more than an hour without the code returning, monitor it closely over the next 24 hours.
Step 2 — Check the flue terminal outside: Go outside and visually inspect the flue outlet (usually a white or grey pipe on an external wall). Ensure it is not blocked by debris, bird nests, snow, or overgrown vegetation. Do not attempt to access the internal flue pipework yourself.
Step 3 — Check your gas supply: Confirm other gas appliances (hob, gas fire) are working normally. If they are not, contact your gas supplier — there may be a supply interruption in your area.
Step 4 — Do not attempt further DIY: Any work beyond the steps above — including removing the boiler casing, testing the sensor, inspecting the heat exchanger, or adjusting gas pressure — must only be carried out by a Gas Safe registered engineer. This is a legal requirement in the UK.
Step 5 — Call a Gas Safe engineer: If the reset does not clear the fault, or the E12 code returns within a short time, book a Gas Safe registered engineer immediately. Do not repeatedly reset a boiler that keeps locking out — this can mask a worsening fault.
📞 When to call a professional
Call a Gas Safe registered engineer if the E12 code returns after a single reset, if the boiler is making unusual sounds before lockout, or if you smell gas at any point (in which case call the National Gas Emergency Service on 0800 111 999 immediately). Tell the engineer you have a Viessmann E12 fault and that you suspect a flue gas sensor or flue restriction issue — this helps them bring the right parts.
⚖️ Repair or replace?
Replacing a flue gas temperature sensor on a Viessmann boiler typically costs £150–£300 including parts and labour. If the heat exchanger is scaled or damaged, repair costs can rise to £400–£600+, at which point replacement becomes worth considering for boilers over 10–12 years old. A new Viessmann boiler costs £1,800–£3,500 installed — if your boiler is ageing and repairs are stacking up, get a replacement quote alongside the repair estimate.