Viessmann bb Fault Code: Causes, Fixes & Repair Costs
What does the Viessmann bb fault code mean?
The BB fault code on Viessmann boilers indicates that the burner has entered a fault or lockout state because the fan was detected as still running when it should have come to a complete stop between heating cycles. Before firing up again, the boiler's control board checks that the fan has fully wound down — if it hasn't, or if the electrical connection to the fan is unreliable, the boiler locks out as a safety precaution. On systems pairing a Vitotronic 300-K with a Vitotronic 200, BB can additionally signal a communication fault between the two controllers. In short, the boiler is refusing to re-ignite because it cannot confirm the fan is properly at rest or correctly connected.
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 fully stopping between cycles Common
After a heating cycle ends, the fan should spin down to a complete standstill before the next ignition attempt. If the fan motor is slow to stop — due to wear, bearing drag, or a sticky rotor — the control board detects rotation when it expects silence and triggers the BB lockout. This is the single most common root cause.
- Loose, corroded, or damaged fan wiring connector Common
The plug-in connector linking the fan to the PCB can work loose over time, develop corrosion, or suffer a damaged pin. An intermittent or absent signal from this connection can mislead the control board into believing the fan is still spinning or is behaving erratically, triggering the same lockout even when the fan motor itself is fine.
- Faulty fan motor Sometimes
If the fan motor itself is failing — worn brushes, degraded bearings, or a damaged tachometer (speed sensor) — it may send incorrect speed readings to the PCB, run at the wrong RPM, or fail to stop cleanly. The boiler treats any of these as an unsafe condition and locks out. A failed fan also compromises flue gas extraction, which is why the boiler won't run without a confirmed, healthy fan signal.
- Flue or air supply restriction Sometimes
A partial blockage in the flue system or the air intake can cause the fan to labour, run unevenly, or take much longer to stop after shutdown. The abnormal fan behaviour that results can then trigger a BB code. Check for obvious external obstructions such as bird nests or debris around the flue terminal.
- PCB fault Rare
If the control board's fan-speed monitoring circuitry is damaged or failing, it may misread a healthy fan as still running, or fail to receive the standstill confirmation signal correctly. PCB faults that produce a BB code are less common but should be considered once the fan, its wiring, and the flue have been ruled out.
- Controller communication fault (Vitotronic 300-K + Vitotronic 200 systems only) Rare
On cascade or zoned systems using a Vitotronic 300-K controller paired with a Vitotronic 200, the BB code can alternatively point to a breakdown in communication between the two controllers rather than a fan issue. An engineer will need to check the data bus connections and controller configuration on these specific installations.
How to fix it
- Check that the gas supply to the boiler is on DIY safe
Confirm the gas isolation valve on the supply pipe to the boiler is fully open (handle in line with the pipe). If other gas appliances in the home are also not working, contact your gas supplier — there may be a wider supply interruption.
- Inspect the flue terminal for obvious external blockages DIY safe
Go outside and check the flue outlet on the external wall or roof. Look for visible obstructions such as bird nests, leaves, or debris covering the terminal grille. Do not insert anything into the flue; simply remove any surface debris from the outside of the terminal if it is safely accessible.
- Attempt a single boiler reset DIY safe
Press and hold the reset button on your Viessmann boiler (refer to your user guide for the exact button location for your model) for roughly 3 seconds. Allow the boiler a full minute to complete its start-up sequence. If it fires up successfully and stays on, monitor it closely over the next few hours. Do not reset more than two or three times in total — repeated resets without a successful start can mask a developing fault or cause further damage.
- Do not open the boiler casing or attempt to access the fan Gas Safe engineer
The fan, its wiring loom, and all associated components are inside the boiler casing. Accessing or adjusting these parts requires Gas Safe registration. Opening the casing yourself could void your warranty, create a safety hazard, and is not permitted under UK gas safety regulations.
- Engineer to inspect and reseat the fan wiring connector Gas Safe engineer
A Gas Safe engineer will remove the casing, locate the plug-in connector between the fan and the PCB, and check it for looseness, corrosion, bent pins, or damaged insulation. Reseating or replacing this connector resolves the fault in a significant number of BB cases and is the first physical check an engineer should carry out.
- Engineer to test the fan motor and tachometer signal Gas Safe engineer
The engineer will power up the fan and monitor its run speed against the boiler's specified RPM range, then confirm it winds down to a complete stop within an acceptable time after shutdown. They will also verify that the tachometer (speed feedback sensor) is returning an accurate signal to the PCB. A fan that fails these checks will need to be replaced.
- Engineer to inspect the flue and air supply pathway Gas Safe engineer
With the boiler open, the engineer can check the internal flue connections, the condensate trap, and the air intake path for partial blockages or restrictions that could be causing the fan to run abnormally. Any blockages found will be cleared and the flue integrity confirmed.
- Engineer to replace the fan if confirmed faulty Gas Safe engineer
If the fan motor is worn, seized, or producing an unreliable speed signal, a like-for-like replacement fan matched to the specific Viessmann model will be fitted. Viessmann fan units vary by boiler series, so the correct part number matters. After fitting, the engineer will commission and test the boiler to confirm the BB fault has cleared.
- Engineer to inspect or replace the PCB if the fan tests healthy Gas Safe engineer
If the fan, its connector, and the flue all check out, the fault lies with the PCB's ability to read the fan signal. PCB replacement is a more involved and costly repair; a competent engineer will confirm this diagnosis before ordering the part to avoid unnecessary expenditure.
- Call a Gas Safe registered engineer if the fault persists after resetting Gas Safe engineer
If the boiler will not stay running after one or two resets, or if BB returns repeatedly, stop attempting resets and book a Gas Safe registered engineer. You can verify an engineer's registration at gassaferegister.co.uk. Continued lockouts mean the underlying fault needs professional diagnosis — further resets will not fix it.
Parts you may need
- Viessmann replacement fan/blower unit · from £175
- Fan wiring harness/plug-in connector · from £25
- PCB (printed circuit board) · from £280
- Flue terminal/grille · 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–£400, depending on the underlying cause.
Frequently asked questions
Is the Viessmann BB fault code dangerous?
The BB code is a safety lockout, which means the boiler has shut itself down in a controlled way rather than running in an unsafe condition. The boiler will not fire while the fault is active, so there is no immediate danger. However, you should not repeatedly reset the boiler trying to force it to run, and you should not attempt to open the casing. Book a Gas Safe engineer to diagnose and clear the fault properly.
Can I fix the BB fault myself by resetting the boiler?
A single reset is worth trying, as an occasional one-off glitch can sometimes clear this way. However, if the BB code returns after one or two resets, resetting again is unlikely to help and the underlying cause — most often a fan motor issue or a loose wiring connector — needs hands-on investigation by a Gas Safe registered engineer. Opening the boiler casing to access the fan yourself is not permitted and could invalidate your warranty.
How much does it cost to fix a Viessmann BB fault in the UK?
For the most common repair — fan inspection, wiring connector check, and fan replacement if needed — most homeowners pay in the region of £150 to £400 including parts and labour. A simple connector reseat at the lower end of that range can sometimes resolve the fault for close to the call-out fee alone. If the PCB turns out to be the cause, replacement typically adds £300 to £500 to the bill, though this is a less frequent outcome. Getting a written quote before authorising parts is always advisable.
Will a BB fault affect my Viessmann warranty?
It depends on your boiler model and service history. The Vitodens 200-W carries a 10-year manufacturer warranty, while the 100-W and 050-W come with 5 years as standard (extendable to 10 years when installed and serviced annually by an approved Viessmann engineer). If your boiler is within warranty and has been serviced every year by a Gas Safe engineer, the repair may be covered. Contact Viessmann UK directly or your installer to check your specific warranty status before commissioning independent repairs.
What is the difference between the BB code on a standard Viessmann boiler and on a Vitotronic 300-K system?
On most Viessmann boilers, BB means the fan has not come to a standstill before re-ignition, or the fan's wiring connection is faulty. On installations that pair a Vitotronic 300-K controller with a Vitotronic 200 — typically found in larger or cascade heating systems — BB can alternatively indicate a communication breakdown between the two controllers rather than a fan problem. An engineer familiar with Viessmann controls will be able to distinguish between the two scenarios by checking the controller data bus connections and the fan diagnostics separately.