What the F4 code means
On the Vitodens 050-W and 100-W, Viessmann's installation and service instructions describe F4 as "there is no flame signal". When the boiler gets a call for heat, the fan starts, the gas valve opens and the ignition electrode sparks. The same electrode then measures a small current through the flame, called the ionisation signal, to confirm the burner is lit. If that signal does not appear, the boiler shuts off the gas and locks out with F4.
On the Vitodens 100-W you will usually see the fault symbol flashing with the letter R, which means the burner has locked out and needs a manual reset. F4 is shared across most current Vitodens models, but always check the fault table in your own manual as older generations can differ.
F4 is a safety lockout. The boiler is stopping unburnt gas from building up, which is exactly what it should do. It does not automatically mean that a part has failed, and very often the cause is outside the boiler.
Common causes of F4
Viessmann's service instructions list the ignition and ionisation electrode and its cable, the gas pressure, the gas train, the ignition module and the condensate drain as the things to check. In practice, that breaks down like this.
No gas or low gas pressure
A closed gas isolation valve, an interruption to the supply or a prepayment meter out of credit will all stop the boiler lighting. Low gas pressure can cause the same result.
Blocked or frozen condensate drain
If condensate cannot drain away, water backs up inside the boiler and affects the burner. In winter a frozen condensate pipe is one of the most common reasons for F4.
Worn or dirty electrode
The ignition and ionisation electrode wears over time and can become dirty or move out of position, so it either fails to light the gas or fails to sense the flame.
Ignition module or cable
A damaged cable or a failing ignition module means the spark never reaches the burner properly.
Gas valve or control fault
If the gas valve does not open correctly, or the control unit is not driving it, the boiler cannot light. These faults need proper testing with the right equipment, so they are always a job for an engineer.
| Cause | Typical sign | Who fixes it |
|---|---|---|
| Prepayment credit run out | Hob will not light either | You, by topping up |
| Gas valve under boiler closed | Handle across the pipe | You, by opening the valve |
| Frozen condensate pipe | Freezing weather, gurgling from boiler | You, by thawing the pipe |
| Worn or dirty electrode | Clicking but no flame, fault returns | Gas Safe engineer |
| Ignition module or cable | No spark heard at start-up | Gas Safe engineer |
| Gas valve or control unit | Gas supply fine, repeated lockouts | Gas Safe engineer |
Safe checks you can do yourself
- Check your other gas appliances. If the hob will not light either, the problem is the gas supply rather than the boiler.
- If you have a prepayment meter, check the credit and top up if needed.
- Check the gas isolation valve on the pipe under the boiler. The handle should be in line with the pipe, which means it is open.
- In freezing weather, find the condensate pipe outside and look for ice. Thaw it with warm (not boiling) water or a hot water bottle. Our frozen condensate pipe guide explains how.
- Check the thermostat and timer are calling for heat.
- Reset the boiler as your manual describes. On the Vitodens 100-W this means pressing and holding R for about two seconds. Stop after two or three attempts.
If you smell gas, do not reset the boiler or use electrical switches. Turn off the gas at the meter if it is safe to do so, open doors and windows, leave the property and call the National Gas Emergency Service on 0800 111 999.
When to call a Gas Safe engineer
If the gas supply is fine, the condensate pipe is clear and F4 comes back after two or three resets, call a Gas Safe registered engineer. They will check the electrode and cable, measure the gas pressure at the boiler, test the gas valve and ignition module, and clear the condensate trap if needed.
When you book, give the code, the model name, whether other gas appliances work and whether the fault began during cold weather. That helps the engineer bring the right parts. Do not remove the casing or try to clean or adjust the electrode yourself, as work on gas components is legally restricted to Gas Safe registered engineers.
Typical repair costs in the UK
If the fix was gas credit or a thawed pipe, there is no cost. If a part is needed, these ranges are typical. Prices vary by region, engineer and model.
| Job | Typical UK cost |
|---|---|
| Engineer call-out and diagnosis | Typically £80 to £150 |
| Ignition and ionisation electrode replacement | Typically £100 to £180 |
| Ignition module replacement | Typically £120 to £220 |
| Gas valve replacement | Typically £250 to £450 |
| Rerouting or insulating the condensate pipe | Typically £100 to £250 |
How to stop F4 coming back
Keep prepayment credit topped up in winter. If your condensate pipe runs outside, ask an engineer whether it can be insulated or rerouted. A yearly service lets an engineer inspect and replace a worn electrode before it causes a breakdown, and our boiler service cost guide explains what is included. For other Vitodens codes, see our Viessmann fault codes page.