Viessmann bC Fault Code: Causes, Fixes & Repair Costs
What does the Viessmann bC fault code mean?
The bC fault code on Viessmann boilers indicates that the boiler's control board has detected an issue with the fan at a moment when it should be completely at rest — typically during the start-up sequence, just before the burner is allowed to fire. The boiler enters a safety lockout and refuses to operate until the condition is resolved. In plain terms, the control board is seeing unexpected signals from the fan (suggesting it is still spinning or sending erroneous readings) or it has identified an unstable plug-in connection somewhere in the fan circuit. Because the fan is a critical combustion-safety component — responsible for safely expelling flue gases from your home — the boiler will not attempt to ignite until it has confirmed the fan is operating correctly. This fault is seen across the Viessmann range, including the Vitodens 050-W, 100-W, 200-W, and 222-F series, though the underlying cause and exact repair procedure can vary slightly by model.
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
- Loose or faulty plug-in connector on the fan wiring harness Common
The bC code description explicitly calls out plug-in connections as a primary cause. Vibration over time, or poor contact from new, can cause the wiring connector between the fan and the PCB to work loose or corrode, sending unreliable speed signals to the control board and triggering the lockout.
- Fan motor wear or failure Common
The fan motor's bearings can deteriorate with age, causing the fan to slow down abnormally or refuse to come to a full stop when commanded. Burnt-out motor windings produce similar symptoms. In either case, the control board detects that the fan is not behaving as expected and raises the bC fault.
- Fan not reaching standstill after previous run Sometimes
If the fan's impeller or motor is partially seized, it may continue to spin — even slowly — after the boiler shuts down. When the boiler next attempts a start-up sequence and checks for standstill, it finds the fan still moving and locks out with the bC code.
- Damaged or shorted wiring between fan and PCB Sometimes
Chafed, pinched, or heat-damaged wiring between the fan assembly and the printed circuit board can cause the control board to receive incorrect or fluctuating voltage signals, making it believe the fan is active when it should not be.
- PCB fault Rare
If the fan and all associated wiring test as healthy, the printed circuit board itself may be misinterpreting or incorrectly commanding the fan circuit. A faulty PCB can generate spurious signals that trigger the bC lockout even when the fan is fine.
How to fix it
- Reset the boiler DIY safe
On most Viessmann models, press and hold the reset button for approximately three seconds (consult your model's handbook for the exact method). Allow the boiler two to three minutes to complete its start-up sequence. If the bC code returns immediately or after one or two firings, do not keep resetting — a recurring fault needs investigation by an engineer, not repeated resets.
- Check the boiler's gas supply DIY safe
Make sure the gas isolation valve on the supply pipe to the boiler is fully open (handle in line with the pipe). Check that other gas appliances in the home — hob, fire — are working normally. If there is no gas at any appliance, contact your gas supplier rather than an engineer.
- Inspect any visible external wiring for obvious damage DIY safe
Without removing any panels or opening the boiler casing, take a visual look at any external cables leading into the boiler. If you can see obvious scorching, fraying, or a connector that has visibly pulled free from its socket on the outside of the casing, make a note to tell the engineer. Do not attempt to open the boiler or reseat internal connectors yourself.
- Contact a Gas Safe registered engineer for a full fan and wiring inspection Gas Safe engineer
An engineer will remove the boiler casing safely and use a multimeter to test the fan's power supply, motor windings, and speed-signal output. They will check every plug-in connector in the fan circuit for corrosion or loose fit and reseat or replace connectors as needed. This is not a task for an unqualified person — the fan sits within the combustion circuit, and incorrect reassembly creates a risk of flue-gas spillage.
- Fan replacement if motor is confirmed faulty Gas Safe engineer
If testing confirms the fan motor has failed — seized bearings, burnt windings, or a speed sensor that cannot be calibrated — the engineer will source and fit a replacement fan assembly compatible with your specific Vitodens or Vitopend model. Because labour costs are similar whether repairing or replacing, a new unit is almost always the recommended approach over attempting to repair the motor.
- PCB testing and replacement if the fan proves serviceable Gas Safe engineer
Should the fan and all wiring test as fully functional, the engineer will turn their attention to the printed circuit board. PCB faults are diagnosed with specialist tools; if the board is confirmed faulty, it will need replacing. Check whether your boiler is still within its Viessmann warranty period — PCB failure on an in-warranty appliance should be reported directly to Viessmann or your registered installer, as it may be covered at no cost to you.
Parts you may need
- Viessmann boiler fan / flue gas blower assembly · from £180
- Fan wiring harness / connector plug · from £25
- Printed circuit board (PCB) · from £320
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 £200–£420, depending on the underlying cause.
Frequently asked questions
Can I reset a Viessmann bC fault myself and will that fix it?
A reset is always worth trying once or twice — occasionally the bC code is triggered by a momentary signal glitch that clears on restart. However, if the fault returns after one or two resets, the underlying cause will not go away on its own. Repeatedly resetting a genuine fan fault means the boiler is continually trying to start without the safety system being satisfied, so it is best to call an engineer at that point.
How much does it cost to fix a Viessmann bC fault in the UK?
For most homeowners the repair involves either a wiring connector fix (roughly £100–£180 all-in) or a fan replacement (typically £200–£420 including parts and labour at 2025 rates). A PCB replacement is a less common outcome but is more expensive — parts alone can be £250–£450, and with labour the total can exceed £550. If your boiler is still under Viessmann warranty, a PCB fault should be covered, so always check before authorising any paid repair.
Is the bC fault dangerous — can I keep using the boiler?
No — the boiler will not fire while the bC lockout is active, which is itself a safety measure. The fan's job is to ensure combustion gases leave your home safely through the flue; if the control board cannot confirm the fan is working correctly, it prevents ignition. Do not attempt to override or bypass the lockout. Leave the boiler off and arrange an engineer visit promptly, particularly in cold weather.
My Viessmann boiler shows bC but the fan sounds like it is running — what does that mean?
This can happen when the fan is spinning but at the wrong speed, or when the fan's speed-sensor signal is corrupted by a faulty connector or damaged wiring. The control board expects to see a specific signal confirming the fan is fully stopped before the start sequence begins; if that confirmation never arrives — even though the fan is broadly operational — it raises the bC fault. An engineer will measure the actual speed-signal voltage to determine whether the motor, the sensor, or the wiring is the root cause.