Boiler
E06
The E06 error code on a Viessmann boiler indicates a fault with the flue gas temperature sensor, which monitors the temperature of exhaust gases leaving the boiler. It’s a moderately serious fault — your boiler will likely lock out and stop working until the issue is resolved.
What does this error mean?
The E06 code tells you that the boiler’s flue gas temperature sensor (also called the exhaust gas sensor) has detected an abnormal reading — either a value that’s too high, too low, or no signal at all. Viessmann boilers use this sensor as a safety check to ensure combustion is working correctly and that flue gases are being expelled safely. When the sensor reports a reading outside its expected range, the boiler shuts down as a precaution to prevent unsafe operation.
Est. repair cost
£150–£300 for sensor replacement by a Gas Safe engineer (parts + labour); new boiler from £1,800–£3,500 installed
🔍 Likely causes
🛠️ Step-by-step fix
Step 1 — Reset the boiler: Press and hold the reset button (usually marked with a flame or ‘R’ symbol) for 3–5 seconds. If the boiler restarts and runs normally, monitor it closely over the next 24 hours for a repeat fault.
Step 2 — Check the condensate pipe: In cold weather, locate the white plastic condensate pipe (usually exits through an outside wall or into a drain) and check it isn’t frozen or blocked. Thaw a frozen pipe using warm — not boiling — water poured over it.
Step 3 — Check the flue terminal outside: Look at where the flue exits your property (typically a white plastic terminal on an outside wall or roof). Ensure it’s not blocked by debris, bird nests, or frost. Do not attempt to dismantle the flue yourself.
Step 4 — Do not attempt further DIY: Any work beyond a reset or condensate thaw on a gas boiler must be carried out by a Gas Safe registered engineer. Touching the flue sensor, wiring, or internal components without Gas Safe registration is illegal and dangerous.
Step 5 — Book a Gas Safe engineer: If the fault returns after a reset or the boiler won’t restart at all, call a qualified engineer and quote the E06 code. They will test the sensor resistance, inspect the wiring loom, and check flue integrity.
📞 When to call a professional
Call a Gas Safe registered engineer immediately if the boiler won’t restart after a reset, or if the E06 code returns repeatedly. Tell them the exact error code (E06), your boiler model (e.g. Viessmann Vitodens 100-W or 200-W), and whether the fault appeared suddenly or has been intermittent. Do not attempt to inspect or replace the flue sensor yourself — this is a gas appliance and all internal work is legally restricted to Gas Safe engineers.
⚖️ Repair or replace?
A flue gas sensor replacement on a Viessmann boiler typically costs £150–£300 all-in for parts and labour. If your boiler is under 10 years old and has been well serviced, repair is almost always the right call. If it’s over 12–15 years old and this isn’t the first fault, get quotes for a new boiler — modern A-rated condensing boilers can cut gas bills significantly and come with manufacturer warranties of up to 10 years.