Boiler
E04
The E04 error code on a Viessmann boiler indicates a fault with the burner ignition — the boiler is attempting to fire but failing to establish or confirm a flame. This is a lockout fault, meaning your boiler has shut itself down as a safety precaution and will not restart without intervention.
What does this error mean?
E04 signals that the boiler’s control board sent the ignition sequence but the ionisation sensor (the component that detects whether a flame is actually burning) did not confirm a stable flame within the permitted timeframe. Viessmann boilers are designed to lock out immediately in this scenario to prevent unburned gas from accumulating. On most Viessmann Vitodens models, E04 specifically points to an ignition failure or flame signal fault rather than a gas supply interruption.
Est. repair cost
£150–£350 for electrode/sensor replacement; £300–£500 for gas valve; £400–£700+ for PCB replacement — all including Gas Safe labour
🔍 Likely causes
🛠️ Step-by-step fix
Step 1 — Reset the boiler: Press and hold the reset button (usually marked with a flame symbol or ‘R’) for 3–5 seconds. Wait 10 minutes and observe whether the boiler fires and the error clears. A single successful reset after an intermittent fault is acceptable; if it locks out again, do not keep resetting.
Step 2 — Check your gas supply: Turn on a gas hob burner to confirm your gas supply is working normally. If there is no gas at other appliances, contact your gas supplier immediately — this is a supply issue, not a boiler fault.
Step 3 — Check the condensate pipe (winter only): Locate the white plastic condensate pipe that exits your boiler and discharges outside or into an internal drain. If it is frozen, carefully pour warm (not boiling) water over the external section to thaw it, then reset the boiler.
Step 4 — Do not attempt further DIY: Beyond the steps above, diagnosing and fixing an E04 fault requires a Gas Safe registered engineer. Do not open the boiler casing, attempt to clean electrodes yourself, or tamper with the gas valve — this is illegal for unqualified individuals and dangerous.
Step 5 — Call a Gas Safe engineer: Book a Gas Safe registered engineer and tell them the exact error code (E04), your boiler model (e.g. Vitodens 100-W, 200-W), and how many times it has locked out. This allows them to bring the most likely replacement parts.
📞 When to call a professional
Call a Gas Safe registered engineer immediately if the boiler fails to stay running after a single reset, or if you smell gas at any point — in the latter case, do not reset the boiler, leave the property, and call the National Gas Emergency Service on 0800 111 999. When booking an engineer, quote the E04 code, your Viessmann model number (found on the boiler data plate inside the front panel), and whether the fault is intermittent or permanent — this significantly speeds up diagnosis.
⚖️ Repair or replace?
For a boiler under 10 years old, repair is almost always the right call: replacing ignition electrodes or the ionisation sensor typically costs £150–£350 all-in, while a gas valve replacement sits at £300–£500 including labour. If your Viessmann boiler is over 12–15 years old and this is not its first major fault, replacing it with a new A-rated condensing boiler (typically £1,800–£3,000 installed) is likely more economical over a 5-year horizon than repeated repairs on an ageing heat exchanger and PCB.