Boiler
E09
The E09 error code on a Viessmann boiler indicates a flame failure or ignition fault — the boiler attempted to light but failed to detect a stable flame. This is a moderately serious fault that will lock your boiler out and leave you without heating or hot water until it’s resolved.
What does this error mean?
Viessmann boilers use an ionisation sensor to confirm that a flame has ignited inside the burner. When the control board fires the igniter but receives no confirmation signal from that sensor, it throws the E09 fault and shuts down as a safety precaution. In plain terms: your boiler tried to light, couldn’t prove it had done so safely, and stopped itself to prevent a gas build-up.
Est. repair cost
£80–£200 for electrode clean/replacement; £200–£500 for gas valve or PCB replacement
🔍 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. Wait 2 minutes and see if the boiler fires up. A one-off lockout after a power cut or gas interruption can often be cleared this way.
Step 2 — Check your gas supply: Turn on a gas hob or check another gas appliance. If nothing is working, your gas supply may be interrupted — contact Cadent Gas (or your regional network operator) before touching the boiler.
Step 3 — Check the condensate pipe (winter months only): Locate the white plastic pipe that exits your boiler and drains outside. If it feels frozen solid, thaw it carefully by pouring warm (not boiling) water along its length, then reset the boiler.
Step 4 — Try a second reset after 10 minutes: If the boiler locked out again immediately after the first reset, wait 10 minutes and try once more. If it locks out a third time, stop resetting — repeated failed ignitions can cause unburnt gas to accumulate.
Step 5 — Call a Gas Safe registered engineer: All remaining causes (dirty electrodes, faulty gas valve, PCB issues) require a qualified engineer. Do not attempt to open the boiler casing or inspect the burner yourself — this is illegal without Gas Safe registration and dangerous.
📞 When to call a professional
If the boiler locks out again after one or two resets, or if you can smell gas at any point, stop immediately, ventilate the room, and call the National Gas Emergency Service on 0800 111 999. For a non-emergency E09 fault that won’t clear, book a Gas Safe registered engineer and tell them: ‘My Viessmann boiler is showing an E09 fault — it’s locking out on ignition failure.’ This helps them come prepared with the right parts.
⚖️ Repair or replace?
A typical E09 repair — cleaning or replacing the ionisation or ignition electrode — costs £80–£200 including labour. A gas valve replacement sits at £200–£400 and a PCB replacement at £250–£500. If your Viessmann boiler is over 12–15 years old and facing a repair above £300, a new boiler (typically £1,800–£3,500 installed in the UK) is worth serious consideration, especially given improved efficiency savings on modern A-rated units.