Boiler fault codes
Viessmann F4 Fault Code: No Flame Detected
Published
Viessmann display
F4
What it means
The boiler tried to fire but no flame signal was detected, so it locked out. Viessmann’s Vitodens 100-W service manual lists the cause of F4 as “no flame signal”.
Safe to check yourself
- Light the gas hob to confirm gas is reaching the property
- On a prepayment meter, check the credit and follow the meter’s own steps to restore supply
- Check the gas isolation valve on the pipe under the boiler is open
- In freezing weather, check the white plastic condensate pipe outside for ice and thaw it with warm, never boiling, water
- Reset once using your model’s reset method, after noting the code
Call an engineer if
- F4 returns after one reset even though the gas and condensate pipe are fine
- No gas appliance in the property will light and the meter has credit (call your supplier or 0800 111 999)
- You see scorch marks, soot or water around the boiler
- You smell gas: open windows, shut off the gas at the meter if safe, keep clear of switches, then get out and call 0800 111 999 from outside
F4 on a Viessmann boiler means no flame signal was detected: the boiler tried to light, the ionisation electrode never confirmed a flame, and it locked out. The usual causes are a gas supply problem, a frozen condensate pipe or a worn electrode. Check the gas and the condensate pipe yourself, reset once, then call an engineer.
What does F4 mean on a Viessmann boiler?
When a Vitodens needs heat, it opens the gas valve, sparks the ignition electrode and then listens for a small current through the flame on the ionisation electrode. That current is the flame signal. If it never arrives, the control unit shuts the gas and locks out rather than keep feeding unburnt gas into the burner. The display shows F4.
Viessmann’s UK service instructions for the Vitodens 100-W give the corrective steps in one line: check the ionisation electrode and cables, the gas pressure, the gas train, the ignition and ignition module, and the condensate drain. Only the gas supply and the outside condensate pipe are checks you can make yourself.
Code lists differ between markets and generations. Viessmann’s North American Vitodens 200-W documentation records a missing flame at start as EE rather than F4, and newer Vitodens models with a black-and-white button display show numbered fault messages. If your boiler shows F4 on a two-character display, the no-flame meaning is the one that applies.
What to check before you reset
Work down this list in order. Most F4 lockouts that clear without a visit are one of the first three.
- Gas to the property: light the hob. If nothing lights anywhere, the fault is the supply or the meter, not the boiler.
- Prepayment meter: a meter out of credit, or one that has used its emergency credit, shuts off the gas. Top up and follow the meter’s restore steps.
- Gas valve under the boiler: if anyone has worked nearby, make sure the isolation valve on the gas pipe is still open.
- Condensate pipe: condensing boilers drain water through a plastic pipe, often to an outside drain. When it freezes, water backs up and the boiler will not fire. Pour warm water along the frozen section.
- Reset: on many Vitodens 100-W models, turn the rotary selector briefly to the reset position and back. On newer models with a button display, follow the reset step in the operating instructions, which usually means holding the marked buttons for a few seconds.
Viessmann warns against resetting fault messages several times in quick succession. If F4 comes back after one reset, stop and call an engineer. If you smell gas at any point, do not reset or use switches; open windows, turn the gas off at the meter if you can reach it safely, leave and call 0800 111 999.
Why does F4 keep coming back?
With the gas on and the condensate clear, a repeat F4 usually means the flame is lighting but the boiler cannot sense it, or it is not lighting at all. The combined ignition and ionisation electrode is a frequent culprit. It wears, carbons up and drifts out of gap. The part number varies by Vitodens generation, so the engineer reads it from the data plate before ordering. It is one of the cheaper parts on the boiler, well below the cost of a gas valve or control board.
Other repeat causes are a damaged ignition lead, a failing ignition module, a sticking gas valve, or low gas pressure reaching the boiler. If the fault appears every cold night and clears by day, the condensate route is the problem, and rerouting or insulating the pipe stops it recurring.
What an engineer does about F4
The engineer measures the gas pressure at the boiler inlet with a manometer. Viessmann’s service manual sets the natural gas working range at 17.4 to 25 mbar and says not to start the boiler below 17.4 mbar but to notify the gas supplier. With the casing off, they inspect the electrode block for wear and gap, check the ignition lead and module, clear the condensate trap and siphon, and finish with a flue gas analyser to confirm clean combustion.
Gas work is carried out by Gas Safe registered engineers. You pay the £60 call-out, and the repair price is agreed on site before any work starts. For overheat lockouts see the Viessmann F2 fault, and if your boiler will not light at all our guide to a boiler not firing up covers the wider picture. Our boiler repairs page explains a visit.
Questions people ask
Can I reset F4 myself?
Yes, once you have confirmed gas is reaching the property and the condensate pipe is not frozen. Reset once. If F4 returns, leave it and call an engineer; repeated resets do not fix an ignition fault.
Is F4 on a Viessmann dangerous?
The lockout is the boiler making itself safe by refusing to keep feeding gas. It becomes an emergency only if you smell gas, in which case open windows, turn off at the meter if safe, get out and call 0800 111 999, or if vulnerable people are without heating in freezing weather.
Why does my Viessmann show F4 only in cold weather?
A frozen condensate pipe is the most likely reason. The water cannot drain, so the boiler will not fire. Thaw the outside section with warm water, reset once, and have the pipe rerouted or insulated if it keeps happening.