Boiler
E01
The E01 error code on a Viessmann boiler indicates a flame failure — the boiler has tried to ignite but cannot detect a stable flame, so it has locked out for safety. This is one of the more common Viessmann fault codes and can range from a minor issue you can resolve with a reset, to a more serious component fault that needs a Gas Safe registered engineer. In this guide, we’ll walk you through exactly what E01 means, the most likely causes, what you can safely do yourself, and when it’s time to pick up the phone.
What does this error mean?
The E01 fault code on Viessmann boilers — including popular models in the Vitodens 050-W, 100-W, and 200-W ranges — is a lockout code triggered by flame failure. In plain English: your boiler attempted to start a heating or hot water cycle, tried to light the burner, but either the flame never appeared or it cut out almost immediately after ignition. Because a boiler cannot safely run without a confirmed flame, the control board shuts everything down and displays E01 to protect you and your home. The boiler’s ionisation electrode (a small probe that detects the presence of a flame by sensing an electrical current through the gas flame itself) has reported back to the control board that no flame is present. This could mean the gas never ignited, the flame was too weak to be detected, or the detection system itself has a fault. Until the underlying cause is resolved, the boiler will remain in lockout and will not restart on its own.
Est. repair cost
£80–£150 for a callout, diagnosis, and basic repair (e.g. electrode replacement); £200–£400+ if the gas valve or PCB requires replacement
🔍 Likely causes
🛠️ Step-by-step fix
Step 1 — Check your other gas appliances: Before touching the boiler, go and test your gas hob or gas cooker. If they also have no gas, the problem is with your gas supply, not the boiler. Check your prepayment meter if you have one, and contact your gas supplier or call the National Gas Emergency Service on 0800 111 999 if you suspect a supply outage.
Step 2 — Check the boiler’s gas service valve: Locate the gas isolation valve on the pipe leading into the boiler (usually a quarter-turn lever or a flat-head screw slot). Make sure it is fully open — the lever should be in line with the pipe, not across it. Do not force it if it feels stiff; leave this to an engineer.
Step 3 — Perform a boiler reset: On most Viessmann Vitodens models, press and hold the reset button (often marked with a flame symbol or the word ‘Reset’) for approximately 3 seconds until the display changes. The boiler will attempt to re-ignite. Stand nearby and listen — a successful ignition will be followed by the sound of the fan and then the burner firing. If the boiler locks out again within a minute or two and redisplays E01, do not keep resetting repeatedly (more than 2–3 attempts can flood the heat exchanger with unburnt gas).
Step 4 — Check for a frozen condensate pipe (in cold weather only): If the outside temperature is at or below freezing, your condensate pipe may be blocked with ice. This pipe is usually a white or grey plastic pipe (typically 21.5mm or 32mm diameter) that exits through an external wall, often low down. You can carefully thaw it by pouring warm (not boiling) water over the external section. Once thawed, reset the boiler as described in Step 3.
Step 5 — Check the boiler’s error history: Many Viessmann Vitodens models allow you to scroll through recent fault codes using the control panel. Refer to your boiler’s manual (or download it free from Viessmann’s UK website using your model number) to access the diagnostics menu. This can help confirm whether E01 is the only fault or whether additional codes are present, which is useful information to pass on to your engineer.
Step 6 — Do not attempt further DIY: All remaining causes of E01 — including inspecting or replacing the ignition electrode, ionisation probe, gas valve, or PCB — require the boiler casing to be removed and involve gas pipework or high-voltage electrical components. These tasks are legally restricted to Gas Safe registered engineers in the UK. Attempting them yourself is dangerous and will invalidate your boiler warranty and home insurance. Stop here and call a professional.
📞 When to call a professional
You should call a Gas Safe registered engineer if: the boiler continues to display E01 after one or two reset attempts; the error returns within a short period of working normally; you have confirmed your gas supply is fine but the boiler still won’t light; or the boiler is making unusual sounds (clicking repeatedly, banging, or humming) during the ignition attempt. When you call, tell the engineer the following: your boiler make and model (e.g. Viessmann Vitodens 100-W), the error code displayed (E01), how long it has been showing, how many times you’ve reset it, and whether you’ve noticed any other symptoms such as strange smells, noises, or whether other gas appliances are working. This will save time on the visit and may allow them to bring the correct parts. Always verify your engineer is Gas Safe registered — you can check this for free at GasSafeRegister.co.uk or by asking to see their Gas Safe ID card before any work begins.
⚖️ Repair or replace?
Whether to repair or replace depends largely on your boiler’s age and the fault identified. If the E01 is caused by a dirty or failed ignition or ionisation electrode, this is a relatively inexpensive repair — these are common service parts for Viessmann boilers and most Gas Safe engineers will carry them. A gas valve replacement is more costly but still worthwhile on a boiler under 8–10 years old. A PCB replacement on an older Vitodens is where the maths starts to shift: PCBs for Viessmann boilers can cost £150–£350 for the part alone, and if your boiler is over 10–12 years old, you may be better investing that money towards a new boiler. As a general rule of thumb used across the UK heating industry: if the repair cost exceeds half the price of a new boiler, and the boiler is over 10 years old, replacement is usually the smarter long-term decision. Viessmann boilers are premium German-engineered units with excellent parts availability in the UK, so a well-maintained boiler under 10 years old is generally worth repairing. New Viessmann Vitodens boilers are typically priced between £1,500 and £3,000 supplied and installed, depending on the model and your property’s requirements.