Boiler
F01
The F01 error code on a Viessmann boiler indicates a flame failure — the boiler is attempting to ignite 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?
F01 means your Viessmann boiler fired up the ignition sequence but the control board received no confirmation from the flame sensor (ionisation electrode) that a flame was successfully established. The boiler performs a set number of ignition attempts, fails to detect a flame each time, and then locks itself out as a safety measure. Until the underlying cause is fixed, the boiler will not restart on its own.
Est. repair cost
£80–£250 professional repair (electrode to gas valve/PCB); no safe DIY repair option for this fault
🔍 Likely causes
🛠️ Step-by-step fix
Check your gas supply first: confirm other gas appliances in your home (hob, gas fire) are working. If they are not, call your gas supplier — this is a supply issue, not a boiler fault.
Press the reset button on your Viessmann boiler (usually a flame symbol or marked ‘Reset’) and hold it for 3–5 seconds. Allow the boiler to attempt ignition. One reset attempt is safe for a homeowner to try.
If the fault returns immediately after reset, do not keep resetting repeatedly — more than two failed attempts in quick succession can indicate a more serious underlying fault and further resets will not help.
Check that the boiler’s gas isolation valve (the lever on the gas pipe near the boiler) is fully open — the handle should be inline with the pipe, not across it.
Check your boiler manual for any additional diagnostic steps specific to your Viessmann model (e.g. Vitodens 100-W, 200-W). Some models display a sub-code alongside F01 that helps pinpoint the component at fault.
If the error persists after one or two resets, stop here and call a Gas Safe registered engineer. Do not attempt to access internal components, adjust gas settings, or clean the electrodes yourself — this is illegal for unqualified persons and dangerous.
📞 When to call a professional
Call a Gas Safe registered engineer if the F01 fault returns after one or two resets, or if you have no fault with your gas supply. Tell the engineer you have a Viessmann boiler showing an F01 flame failure lockout — mention your exact model if you know it (e.g. Vitodens 100-W) and how many times you have tried resetting. Do not attempt any internal work yourself; working on a gas appliance without Gas Safe registration is illegal under UK law and extremely dangerous.
⚖️ Repair or replace?
A professional repair for an F01 fault typically costs £80–£250 depending on the component replaced — an electrode clean or replacement sits at the lower end, while a gas valve or PCB sits at the higher end. If your Viessmann boiler is under 10 years old and has been reliable, repair is almost always the right call. If the boiler is over 12–15 years old and facing its second or third major fault, a replacement boiler (£2,000–£3,500 installed) is likely the more cost-effective long-term decision.