What the FD code means
The burner control unit is the electronic brain that runs the ignition sequence, watches the flame and controls the fan and gas valve. On the Vitodens 100-W, Viessmann's installation and service instructions describe FD as a "burner control unit fault", with the burner blocked. Depending on the display, the code may show as "Fd" in mixed case.
FD is a broad code. It tells you the control unit is unhappy, but not always why. Sometimes the control unit really has failed. Other times it has been upset by something outside it, such as a problem with the ignition electrodes and cables or electrical interference from nearby equipment. That is why Viessmann's steps start with those checks before replacing the control unit.
Viessmann also lists FF with the same description and the same checks, so if your boiler has shown FF in the past, the advice here applies too.
Because the code is general, the pattern matters. A single FD that clears with one reset is often a one-off disturbance. An FD that returns every day, or every time the boiler starts, points to a component that needs attention. Try to note when it appears: at start-up, while the boiler is running, or at random times. Faults that happen at the moment of ignition lean towards the electrodes and ignition cables, while random faults with no obvious trigger lean more towards interference or the control unit itself.
Common causes of FD
Ignition electrode or cable problems
Viessmann's first check is the ignition electrodes and their connecting cables. A worn electrode or a damaged ignition cable can disturb the control unit during ignition.
Electrical interference
Viessmann specifically mentions checking whether a strong electromagnetic interference field exists near the appliance. Powerful electrical equipment close to the boiler, or wiring faults, can interfere with the electronics.
A one-off glitch
A power interruption or a single disturbance can trigger FD once. If a reset clears it and it does not return, this may be all it was. Storms and brief outages are common triggers for one-off faults like this.
Failed control unit
If the electrodes and cables are fine and there is no interference, Viessmann's instruction is to replace the control unit if the fault persists. Water getting onto the electronics, for example from a leak inside the boiler, can also damage the control unit.
| Cause | Typical sign | Who fixes it |
|---|---|---|
| One-off glitch or power blip | Fault clears after one reset | You, by resetting |
| Nearby electrical interference | Fault started after new equipment was fitted nearby | Gas Safe engineer or electrician |
| Ignition electrode or cable | Fault around start-up, clicking sounds | Gas Safe engineer |
| Failed control unit | Fault returns, other checks fine | Gas Safe engineer |
Safe checks you can do yourself
- Check the fused spur and power supply to the boiler, and note any recent power cuts.
- Think about whether anything electrical has recently been installed close to the boiler, such as new appliances, chargers or other equipment. Make a note for the engineer rather than moving wiring yourself.
- Check the area around and under the boiler for signs of water, which could point to a leak affecting the electronics. Our guide on a boiler leaking water explains what to look for.
- Check your thermostat and timer are calling for heat.
- Reset the boiler as your manual describes. On the Vitodens 100-W, press and hold R for about two seconds. Stop after two or three attempts.
Do not remove the casing or touch the control unit, electrodes or wiring. The control unit manages the gas valve and ignition, so any work on it must be done by a Gas Safe registered engineer. If you smell gas, call the National Gas Emergency Service on 0800 111 999.
When to call a Gas Safe engineer
If FD returns after two or three resets, or appears alongside other codes such as F4 or F9, call a Gas Safe registered engineer. They will check the ignition electrodes and cables, look for interference or wiring problems near the boiler, and check for water damage to the electronics before deciding whether the control unit needs replacing.
When you book, give the code, the model name and any other codes you have seen. Mention any new electrical equipment near the boiler and any recent electrical work in the house. Because a new control unit is one of the more expensive parts, it is worth making sure the simpler causes have been ruled out first.
Typical repair costs in the UK
If a reset clears FD and it does not come back, there is no cost. If a repair 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 electrode replacement | Typically £100 to £180 |
| Ignition cable replacement | Typically £80 to £150 |
| Control unit replacement | Typically £250 to £450 fitted |
How to stop FD coming back
Keep powerful electrical equipment away from the boiler where you can, and make sure any leaks are dealt with quickly so water never reaches the electronics. A yearly service lets an engineer replace a worn electrode before it causes problems, and our boiler service cost guide explains what that typically costs. If your boiler shows a fan fault, see our page on Viessmann F9, and for a no flame fault see Viessmann F4.