Viessmann FC Fault Code: Causes, Fixes & Repair Costs
What does the Viessmann FC fault code mean?
The FC fault code on a Viessmann boiler signals that the burner has been forced into fault mode — typically because the fan has not come to a complete standstill when it should have, or because a plug-in connection within the boiler is unstable or intermittent. The boiler's PCB has detected a condition it considers unsafe and has locked the unit out to prevent further operation. On the Vitodens 100-W specifically, this fault often presents alongside the DIP switch S2 being set to the ON position. Because the FC code can indicate a genuinely dangerous situation — such as combustion gases not being safely vented — you should not attempt to reset the boiler repeatedly and should call a Gas Safe registered engineer promptly.
General guidance only — not a substitute for professional advice, and not certified by a Gas Safe engineer. Always have your specific appliance assessed by a qualified professional. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- Fan fault or failure Common
The boiler's fan creates the draught needed to push combustion gases safely out through the flue. If the fan is running too slowly, has seized, or is drawing no power at all, the PCB will trigger the FC fault to prevent unsafe venting. This is the most frequently diagnosed cause.
- Loose or faulty plug-in connection Common
Internal wiring connectors — particularly those linking the fan, gas valve, or PCB — can work loose over time due to vibration or thermal cycling. An unstable connection can cause the PCB to misread component signals and lock the boiler out with an FC code.
- Gas valve problem Sometimes
A closed gas shut-off valve, a faulty gas pressure switch, or internal gas valve failure can all contribute to the FC fault. The PCB may receive incorrect or no feedback from the valve, interpreting this as a burner fault condition.
- Blocked or incorrectly installed flue Sometimes
A partial blockage or installation defect in the flue system can restrict the safe discharge of exhaust gases, preventing the fan from reaching or maintaining the correct speed and triggering the fault.
- PCB fault Rare
In some cases the PCB itself is faulty rather than any of the components it monitors. A failing PCB may generate spurious FC codes even when the fan and gas valve are functioning correctly. Engineers will test the PCB and all associated wiring before confirming this diagnosis.
How to fix it
- Check that your gas supply is active DIY safe
Verify that other gas appliances in your home — such as a hob or gas fire — are working normally. If nothing gas-powered is working, contact your gas supplier rather than your boiler engineer.
- Do not repeatedly reset the boiler DIY safe
Unlike some boiler faults, the FC code can indicate a potentially hazardous condition. Avoid pressing the reset button more than once. If it relocks immediately, leave it and call an engineer.
- Check system pressure DIY safe
Look at the pressure gauge on the boiler. It should read between 1 and 1.5 bar when cold. If it is below 1 bar, top it up via the filling loop following your boiler's user guide. Low pressure alone is unlikely to cause an FC fault, but it rules out a secondary issue.
- Do not open the boiler casing or touch internal components Gas Safe engineer
Inspecting or adjusting the fan, gas valve, PCB, wiring, or any internal part must only be carried out by a Gas Safe registered engineer. It is illegal and dangerous for an unqualified person to work on gas appliances.
- Have a Gas Safe engineer inspect the fan and wiring Gas Safe engineer
An engineer will use a multimeter to test whether the fan is receiving power and running to specification. They will also check all plug-in connectors and wiring harnesses for signs of looseness, corrosion, or damage, repairing or replacing as necessary.
- Have the engineer test the gas valve and pressure switch Gas Safe engineer
The engineer will confirm the gas shut-off valve is fully open, test the pressure switch, and assess the gas valve's electrical and mechanical operation, replacing the valve if it is found to be defective.
- Have the engineer assess the flue system Gas Safe engineer
The engineer will inspect the flue for blockages, incorrect pitch, or installation faults that could restrict exhaust gas flow and cause the fan to underperform.
- Have the PCB tested if all other components check out Gas Safe engineer
If the fan, valve, flue, and wiring all test satisfactorily, the engineer will assess the PCB itself. A faulty PCB will need to be replaced with a Viessmann-compatible unit and reprogrammed accordingly.
Parts you may need
- Boiler fan (flue gas blower) · from £150
- Gas valve assembly · from £180
- PCB (printed circuit board) · from £280
- Wiring harness / plug-in connectors · from £40
- Flue gas pressure switch · from £35
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £150–£450, depending on the underlying cause.
Frequently asked questions
Is it safe to keep using my boiler when the FC fault code is showing?
No. The FC code causes the boiler to lock out precisely because the PCB has detected a potentially unsafe condition — most often relating to the fan or a wiring fault that could affect safe venting of combustion gases. Leave the boiler switched off and arrange for a Gas Safe registered engineer to attend as soon as possible.
Can I reset my Viessmann boiler to clear the FC fault?
You can try a single reset, but if the boiler relocks straight away — or if you are unsure why the fault appeared — do not keep resetting it. Repeatedly forcing the boiler to restart when an FC fault is present could be dangerous. The underlying cause needs to be diagnosed and repaired by a qualified engineer.
How much does it typically cost to fix a Viessmann FC fault?
Most FC fault repairs fall in the range of £150 to £450, covering labour plus the cost of the most commonly replaced parts: the fan (roughly £250 all-in), wiring repairs (£60–£120), or a gas valve (£230–£420). If the PCB turns out to be the culprit, expect to pay £400–£500 for the part alone, plus labour. In rare cases where the boiler is beyond economic repair, a full replacement starts at around £2,100. Getting a written diagnosis before authorising any major part replacement is always advisable.
Why does the Vitodens 100-W show FC with DIP switch S2 set to ON?
On the Vitodens 100-W, the FC fault is sometimes accompanied by DIP switch S2 being in the ON position. This configuration can affect how the control board interprets fan and burner signals. A Viessmann-trained installer will check the DIP switch settings as part of their diagnosis, since an incorrect switch position can itself contribute to the fault or affect how the repair is carried out.
Will an annual service help prevent the FC fault code?
Yes. A yearly service by a Gas Safe engineer gives them the opportunity to spot loose wiring connections, a fan beginning to wear, or early signs of gas valve deterioration before they trigger a lockout. For Viessmann boilers with a stainless steel Inox-Radial heat exchanger, the service must include a full combustion analysis — not just a visual check — to keep the manufacturer's 10 or 12-year warranty valid.