Boiler
F03
The F03 error code on a Viessmann boiler indicates a fault with the ignition process — the boiler is attempting to fire but failing to detect a stable flame. This is a moderately serious fault that will lock out your boiler and leave you without heating or hot water until it is resolved.
What does this error mean?
F03 signals that the boiler’s control board has attempted ignition and either no flame was established, or the flame sensor (ionisation electrode) could not confirm one within the permitted number of tries. As a safety measure, the boiler locks out to prevent unburned gas from accumulating. This is not a fault you can ignore — an ignition failure must be investigated and fixed before the boiler will restart reliably.
Est. repair cost
£150–£300 for electrode or sensor replacement; £400–£600 for gas valve or PCB; new boiler £2,000–£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. Wait 2 minutes for a full restart attempt. If the boiler fires successfully, monitor it over the next 24 hours.
Step 2 — Check your gas supply: Turn on a gas hob burner or another gas appliance. If it also fails to ignite or has weak flame, contact your gas supplier — the issue is with your supply, not the boiler.
Step 3 — Check the condensate pipe (in freezing weather): A frozen condensate pipe can cause ignition lockouts. Locate the white plastic pipe exiting your boiler to an outside drain and pour warm (not boiling) water over any frozen section, then reset.
Step 4 — Do not attempt further DIY: All remaining causes — cleaning the ionisation electrode, replacing the gas valve, inspecting the burner, or testing the PCB — require a Gas Safe registered engineer. Do not remove any boiler casing or interfere with gas components.
Step 5 — Call a Gas Safe engineer and report the F03 code: Tell them the exact error code, how many times it has locked out, and whether the reset holds. This helps them bring the right parts to the first visit.
📞 When to call a professional
If the boiler locks out again within 24 hours of a successful reset, call a Gas Safe registered engineer immediately — do not keep resetting repeatedly, as this can mask a worsening fault. Tell the engineer you have a Viessmann F03 lockout and describe how many ignition attempts have failed. Any work beyond a reset is legally required to be carried out by a Gas Safe registered professional in the UK.
⚖️ Repair or replace?
A typical F03 repair — replacing the ignition electrode or flame sensor — costs £150–£300 including labour in the UK. A gas valve or PCB replacement can reach £400–£600. If your Viessmann boiler is over 10–12 years old and repair costs exceed £400, a new boiler (typically £2,000–£3,500 installed) is often better value and comes with a manufacturer warranty. For boilers under 8 years old, repair is almost always the right call.