Boiler
E08
The E08 error code on a Viessmann boiler indicates a fault with the ignition or flame detection circuit — the boiler has attempted to light but cannot confirm a stable flame. This is a safety lockout and your boiler will stop working until the fault is resolved.
What does this error mean?
E08 signals that your Viessmann boiler’s control board has tried to ignite the burner but the ionisation sensor — the component that confirms a flame is present — has not detected a successful ignition. The boiler locks out as a precaution to prevent unburned gas from accumulating. This is a safety feature, not a nuisance fault, and should be taken seriously.
Est. repair cost
£150–£350 for a Gas Safe engineer call-out, diagnosis, and parts (ionisation electrode or gas valve). A PCB replacement can reach £400–£600.
🔍 Likely causes
🛠️ Step-by-step fix
Check your gas supply first: confirm other gas appliances (hob, gas fire) are working. If they’re not, contact your gas supplier — this is not a boiler fault.
Inspect the condensate pipe (the white plastic pipe that usually exits through an outside wall or into a drain). In freezing temperatures, thaw it with warm — not boiling — water poured along its length, then attempt a reset.
Reset the boiler: locate the reset button (usually marked with a flame symbol or ‘R’) and hold it for 3–5 seconds. Consult your Viessmann manual for the exact procedure for your model. Allow the boiler 2–3 minutes to attempt ignition.
If the E08 returns after one or two resets, do not keep resetting repeatedly — this can flood the heat exchanger with unburned gas. Stop here and call a Gas Safe registered engineer.
Do NOT attempt to inspect or clean the ignition electrodes, gas valve, or any internal gas components yourself. Working on gas appliances without Gas Safe registration is illegal and dangerous.
📞 When to call a professional
Call a Gas Safe registered engineer if the E08 code returns after a single reset attempt, or if you cannot identify an obvious external cause such as a frozen condensate pipe or gas supply interruption. Tell the engineer you have a Viessmann E08 lockout fault and that you suspect an ignition or flame detection issue — this helps them bring the right parts. Do not attempt further resets or open the boiler casing.
⚖️ Repair or replace?
For a boiler under 10 years old, repair is almost always the right call — replacing an ionisation electrode or gas valve typically costs £150–£350 all-in, which is well worth it on a quality Viessmann unit. If your boiler is over 12–15 years old and this isn’t its first major fault, a new A-rated condensing boiler (£1,800–£3,500 installed) will be more cost-effective over the next decade. Viessmann boilers have a strong reputation for longevity, so don’t write one off prematurely.