Boiler
E05
The E05 error code on a Viessmann boiler indicates a flue gas temperature sensor fault — it’s a safety-related fault that will lock out your boiler and should not be ignored.
What does this error mean?
The E05 code means your Viessmann boiler’s flue gas temperature sensor (also called the exhaust thermistor) has detected a reading outside its safe operating range, or the sensor itself has failed. The boiler shuts down as a precaution to prevent overheating or unsafe flue conditions. This is a built-in safety lockout — the boiler is doing exactly what it’s designed to do.
Est. repair cost
£150–£300 for a Gas Safe engineer to diagnose and replace the flue temperature sensor, including parts and labour
🔍 Likely causes
🛠️ Step-by-step fix
Step 1 — Perform a manual reset: Press and hold the reset button on your Viessmann boiler for 3–5 seconds. If it fires up and runs normally for more than 30 minutes without returning the E05 code, monitor it closely over the next 24 hours.
Step 2 — Check your flue outlet externally: Go outside and visually inspect the flue terminal (usually a white plastic vent on an external wall or roof). Ensure it’s not blocked by leaves, a bird nest, frost, or debris. Do not attempt to dismantle anything — this is a visual check only.
Step 3 — Check for adequate ventilation: If your boiler is in a cupboard or utility room, ensure vents and air bricks in the room are clear and unobstructed.
Step 4 — Do not attempt to access the sensor, wiring, or heat exchanger yourself. These components are internal to a gas appliance and working on them requires Gas Safe registration. Stop here and call a qualified engineer.
Step 5 — If the error returns after reset, lock out the boiler by switching it off at the programmer or thermostat and arrange an engineer visit promptly. Do not keep resetting repeatedly — persistent lockouts indicate a real fault.
📞 When to call a professional
Call a Gas Safe registered engineer if the E05 error returns after a single reset, or if the boiler won’t reset at all. Tell them: ‘My Viessmann boiler is showing an E05 fault code — I believe it’s a flue gas temperature sensor issue.’ This helps them arrive with the right parts. Do not attempt to open the boiler casing or touch any internal components — this is illegal for unqualified individuals on gas appliances and genuinely dangerous.
⚖️ Repair or replace?
A flue gas sensor replacement by a Gas Safe engineer typically costs £150–£300 including parts and labour. If your Viessmann boiler is under 10 years old, repair is almost always worthwhile — Viessmann boilers are well-built and have long service lives. If your boiler is over 15 years old and has had multiple faults, a new boiler (typically £2,000–£3,500 installed for a Viessmann Vitodens) may be more economical long-term.