Viessmann bF Fault Code: Causes, Fixes & Repair Costs
What does the Viessmann bF fault code mean?
The bF fault code on Viessmann boilers indicates that the boiler's control unit has detected the fan was not at a complete standstill when it should have been, or that there is an unstable plug-in connection on the fan circuit. As a safety response, the boiler enters a protective lockout and prevents the burner from firing. The fan plays a critical role in safely expelling combustion gases, so any sign that it is misbehaving — whether it is still spinning at the wrong time, running at the wrong speed, or has a loose electrical connection — causes the boiler to shut down rather than risk unsafe operation.
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 not reaching standstill after shutdown Common
After the boiler completes a heating cycle and begins to wind down, the fan should come to a complete stop within a set time window. If the fan continues spinning — due to worn bearings, a damaged motor, or a sticking impeller — the control board sees this as a fault and locks the burner out on the next start attempt. Incorrect fan RPM readings can also trigger this even when the physical rotation appears normal.
- Loose or faulty plug-in connector on the fan Common
The fan is connected to the boiler's wiring loom via a plug-in connector. Over time, vibration, heat cycling, and general wear can cause this connector to become loose, corroded, or intermittently disconnected. The control board then loses a reliable signal from the fan and raises the bF code as a precaution. This is a straightforward fix for an engineer but cannot safely be investigated without opening the boiler casing.
- Worn or failing fan motor Sometimes
The fan motor itself can degrade over time, particularly in boilers that have been in service for several years. Bearings wear out, windings deteriorate, and the motor may begin drawing the wrong current or failing to reach the correct speed. The boiler's control unit monitors fan performance and will lock out if the motor is no longer behaving within expected parameters.
- PCB misreading fan signals Rare
The printed circuit board (PCB) receives and interprets feedback from the fan. If the PCB develops a fault — due to moisture ingress, age-related component failure, or electrical surge damage — it may incorrectly report the fan as still running or as having a connection problem, even when the fan itself is physically fine. This is a less common cause but should be considered if the fan and wiring check out correctly.
How to fix it
- Reset the boiler once or twice DIY safe
On many Viessmann models, you can reset the boiler by turning the heating mode dial to the reset symbol and then quickly returning it to your desired setting. On some models with a digital interface, hold the 'Mode' and 'OK' buttons simultaneously for a few seconds. If the bF code clears and the boiler runs normally, monitor it closely. If the fault returns after one or two resets, do not keep resetting — a recurring lockout means the underlying problem needs professional diagnosis.
- Check that your gas supply is on DIY safe
Before calling an engineer, quickly confirm that your gas supply has not been interrupted. Check that other gas appliances in the home (such as a hob or gas fire) are working normally. If none of your gas appliances are working, contact your gas supplier rather than a boiler engineer.
- Contact a Gas Safe registered engineer for a fan and wiring inspection Gas Safe engineer
A qualified engineer will open the boiler casing and carry out a thorough inspection. They will test the fan motor for correct speed and current draw, check the blades and bearings for physical damage, and reseat or replace the plug-in connector on the fan circuit. They will also inspect the fan wiring loom for signs of corrosion, chafing, or heat damage. Do not attempt to open the boiler casing or touch internal components yourself — this is unsafe and will void your Viessmann warranty.
- Have the engineer test the PCB if the fan is found to be in order Gas Safe engineer
If the fan, its motor, and all wiring connections are confirmed to be in good condition, the engineer will then assess whether the PCB is misinterpreting the fan feedback signal. PCB diagnosis requires specialist equipment and an understanding of the boiler's control logic. If the PCB is found to be faulty, the engineer will advise on repair or replacement. On older boilers, always weigh the cost of a PCB replacement against the cost of a new boiler installation before proceeding.
Parts you may need
- Boiler fan / flue fan assembly · from £160
- Fan plug-in wiring connector / harness · from £35
- PCB (printed circuit board) · from £280
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 £120–£350, depending on the underlying cause.
Frequently asked questions
Is the Viessmann bF fault dangerous?
Not immediately — the boiler has entered a protective lockout state specifically to prevent unsafe operation. This means the burner is not firing and combustion gases are not being produced. However, you should not repeatedly reset the boiler and ignore the fault, as the underlying cause needs to be properly diagnosed and resolved by a Gas Safe registered engineer.
Can I fix the bF fault myself?
The only homeowner-safe action is to attempt one or two resets as described in your boiler manual. Everything beyond that — including inspecting the fan, wiring connectors, or PCB — requires opening the boiler casing, which should only be done by a Gas Safe registered engineer. Attempting internal repairs yourself is unsafe, likely illegal, and will invalidate your Viessmann warranty.
How much does it cost to fix a Viessmann bF fault code?
If a loose wiring connector is to blame, an engineer can often resolve it in a single visit for around £120–£180 including call-out. A fan replacement typically costs £200–£350 depending on the specific Viessmann model and regional labour rates. If the fault turns out to be a failed PCB, costs can rise to around £450 or more including parts and labour. On older boilers, it is worth comparing that figure against the cost of a new boiler before committing to a PCB replacement.
Will the bF fault affect my Viessmann warranty?
Viessmann's extended warranties (which can run to 10 or 12 years) require annual servicing by a Gas Safe registered engineer who checks combustion levels, the burner gasket, and ionisation readings. If your boiler has not been serviced regularly, a warranty claim relating to this fault may not be honoured. Always use a Gas Safe engineer for both servicing and fault repairs to protect your warranty entitlement.