Viessmann bA Fault Code: Causes, Fixes & Repair Costs
What does the Viessmann bA fault code mean?
The bA fault code appears on Viessmann boilers when the burner has entered a locked-out error state because the fan was detected as still spinning at a point when it should have come to a complete standstill — typically just before the boiler attempts a fresh start cycle. It can also trigger when the control unit picks up an unstable or unreliable signal from the fan's plug-in connector. Because the fan is a core safety component — responsible for expelling combustion gases safely out through the flue — the boiler refuses to fire until it can confirm the fan is behaving correctly. A manual reset is required before the burner can attempt to restart, but if the code returns after resetting, an engineer needs to investigate the fan, its wiring, and the control electronics.
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
- Faulty or failing fan motor Common
The fan motor itself may have worn bearings, a seized rotor, or an internal electrical fault causing it to run erratically, fail to stop on command, or send a garbled speed signal back to the PCB. Because the boiler relies on a confirmed fan standstill before it will restart, even a briefly misbehaving fan triggers a full lockout.
- Loose or damaged plug-in connection at the fan Common
Viessmann fans connect to the wiring harness via a plug-in adaptor. Vibration over time can loosen this connector, and heat cycling can cause pin corrosion. A poor connection gives the control board an intermittent or absent signal, which it interprets the same way as a fan that isn't stopping — leading to the bA code appearing even when the fan itself is mechanically fine.
- Incorrect fan RPM — fan not idling after auto-reset Sometimes
After the boiler completes an auto-reset attempt, the fan should slow to a full stop within a set window. If the fan's speed sensor (tacho) reports that the blades are still turning above the expected threshold, the control board refuses to proceed and latches the bA fault. This can be caused by a worn tacho sensor or a fan that is simply taking too long to decelerate.
- PCB or burner control unit fault Sometimes
If the fan and all its wiring check out as healthy, the problem may lie in the PCB itself misreading the fan status signal. A failing control board can incorrectly conclude the fan is still running when it has already stopped, or fail to process the standstill confirmation correctly, resulting in a persistent bA code that survives multiple resets and fan replacements.
- Blocked flue or restricted airflow Rare
A partially blocked or incorrectly routed flue can create back-pressure that slows fan deceleration or causes the blades to keep moving after power is cut. Debris, bird nests, or condensate build-up at the flue terminal are typical culprits. This is more likely to contribute to the fault in combination with another issue rather than trigger it in isolation.
How to fix it
- Reset the boiler once or twice DIY safe
Press and hold the reset button on your Viessmann boiler for a few seconds until the display clears and the boiler attempts to restart. A single transient signal glitch can occasionally cause the bA code, and a reset may clear it. Do not reset more than two or three times in quick succession — repeated resets without a fix can mask a developing fault and, in rare cases, mask a safety issue.
- Check the flue terminal outside your property DIY safe
Go outside and take a look at the point where the flue pipe exits your wall or roof. Make sure it isn't visibly blocked by debris, a bird nest, frost, or anything else restricting airflow. Do not attempt to dismantle any part of the flue — this is just a visual check. If you spot an obvious blockage you cannot safely remove yourself, mention it to the engineer.
- If the fault returns, call a Gas Safe registered engineer Gas Safe engineer
A returning bA code means the boiler has detected a genuine fan or connection problem that a reset cannot fix. At this point, a qualified engineer needs to attend. They will inspect the fan motor for signs of wear or seizure, check the plug-in connector and wiring harness for corrosion or loose pins, and verify the fan's tacho signal is reaching the PCB correctly.
- Engineer: inspect and test the fan motor and wiring Gas Safe engineer
The engineer will power down the boiler, remove the casing, and manually check that the fan spins freely with no resistance. They will reseat or replace the plug-in connector, check the wiring harness for damage, and use diagnostic tools to confirm the tacho signal is clean and within specification. In many cases, reseating a loose connector resolves the fault.
- Engineer: replace the fan unit if required Gas Safe engineer
If the fan motor is found to be faulty — worn bearings, a failed tacho, or intermittent running — the engineer will fit a replacement fan unit. Because labour costs make up a large portion of the bill, repair of individual fan components is rarely cost-effective compared to fitting a new unit. Only a Gas Safe registered engineer should carry out this work.
- Engineer: assess and replace the PCB if the fault persists Gas Safe engineer
Should the new or inspected fan check out as fully functional but the bA code continues to appear, the control board is the likely culprit. The engineer may test it with diagnostic equipment or substitute a known-good unit. PCB replacement is a more involved and more expensive repair, so a thorough fan investigation should always come first.
Parts you may need
- Viessmann boiler fan/flue motor assembly · from £120
- Fan plug-in connector and wiring harness · from £25
- Fan tacho (speed sensor) · from £35
- Viessmann PCB / burner control unit · from £200
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–£350, depending on the underlying cause.
Frequently asked questions
Can I fix the Viessmann bA fault code myself?
The only safe DIY steps are attempting one or two resets and doing a quick visual check of the outdoor flue terminal. Everything beyond that — inspecting the fan, testing the wiring, or replacing any components — must be done by a Gas Safe registered engineer. The fan is directly linked to safe combustion gas removal, so incorrect repairs can create a carbon monoxide risk and will almost certainly void your Viessmann warranty.
How much does it cost to fix the bA fault code in the UK?
Most people with this fault pay somewhere between £150 and £350 all-in, covering the engineer's time and parts. A straightforward fix like reseating a loose connector or a minor repair sits toward the lower end. Replacing the fan motor with a new unit typically costs around £250–£300 including labour. If the PCB turns out to be the root cause, that repair can push costs to £400–£500 or beyond — but PCB replacement is far less common than a fan fault for this code.
Why does my Viessmann boiler keep showing bA even after resetting?
If the bA code returns each time you reset, the boiler has confirmed that something is genuinely wrong rather than a one-off signal glitch. The most common reason is a fan motor that is failing or a plug-in connection that is loose enough to give unreliable readings. Repeated resetting without a fix will not resolve the underlying fault — you need an engineer to test the fan and its wiring before the boiler will stay clear of the lockout.
How can I tell whether the bA fault is the fan or the PCB?
You cannot reliably distinguish between the two without diagnostic equipment, which is why an engineer's assessment is essential. In practice, the fan (or its connector) is the culprit in the large majority of bA cases. Engineers typically confirm the fan and wiring are healthy before considering the PCB — if a replacement fan clears the code, the PCB was never the issue. Only when a confirmed good fan still results in the same lockout does PCB replacement become the logical next step.