Buderus 3A Fault Code: Causes, Fixes & Repair Costs
What does the Buderus 3A fault code mean?
Fault code 3A on Buderus boilers points to a problem with the flue fan (also called the blower). Depending on the model, the boiler's control board has either detected that the fan has stopped running, lost the tacho (speed) signal from the fan during normal operation, or cannot detect the fan sensor at all. On Logamax Plus models this is described as 'fan sensor not detected', while on the 500-series (Boulter Buderus) the service manual refers specifically to the tacho signal failing during the operating phase. On some newer models such as the GB172iT and GB192i, code 3A can also indicate that the differential pressure switch has opened during operation. Whatever the specific trigger, the boiler's control board shuts the burner down as a safety measure — without a working fan to expel combustion gases through the flue, it is unsafe for the boiler to fire.
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
- Failed fan motor or worn bearings Common
The fan motor itself is the most frequent culprit. Over time, the motor windings can burn out or the bearings deteriorate, causing the fan to seize or run too slowly to generate a valid tacho signal. The boiler immediately locks out when this is detected.
- Faulty tacho sensor or magnet Common
The tacho signal is generated by small magnets on the fan rotor passing a sensor. If one of those magnets is damaged or missing, the fan may spin freely but the control board receives no speed signal and treats it as a fan failure. The fan will also stall if it stops on a faulty magnet, preventing restart.
- Loose or damaged fan wiring Sometimes
A loose plug, corroded connector, or damaged cable between the fan and the PCB (UBA controller) can interrupt the tacho signal without the fan motor itself being at fault. An engineer will check continuity through the wiring harness back to the board connector.
- Blockage or debris in the fan or flue Sometimes
Dust accumulation on the fan blades, or a partial blockage in the flue or air intake, can overload the motor and cause it to stall. Checking the fan assembly for dirt and confirming the flue is clear is part of any diagnostic.
- Differential pressure switch fault (applicable models) Rare
On models like the GB172iT and GB192i, code 3A can be triggered by the differential pressure switch opening during operation rather than a direct fan fault. The switch, its connecting hoses, and associated wiring should all be inspected by an engineer.
How to fix it
- Check that other gas appliances in your home are working DIY safe
Before assuming a boiler fault, confirm your gas supply is live by checking whether a gas hob or other appliance works normally. If there is no gas supply, contact your gas supplier rather than resetting the boiler.
- Attempt a single reset — no more than two or three times in total DIY safe
Press and hold the reset button for 2–3 seconds as described in your boiler's user guide. If the boiler fires and code 3A does not return, monitor it for the next hour. If the fault reappears immediately or comes back within a short time, do not keep resetting — repeated resets risk damaging the ignition transformer and will not cure an underlying component failure. Stop after two or three attempts.
- Call a Gas Safe registered engineer to carry out a diagnostic inspection Gas Safe engineer
An engineer will remove the boiler casing (this breaks the safety room seal and must only be done by a qualified person), inspect the fan visually for signs of seizure or debris, and check the blower for dirt and blockages. They will also inspect the fan plug and cable for corrosion or damage and test continuity through the wiring harness back to the UBA control board.
- Engineer to test the fan motor and tacho sensor Gas Safe engineer
Using appropriate test equipment, the engineer will check whether the fan motor runs and whether the tacho signal is being generated correctly. They will also check the condition of the rotor magnets. If the wiring harness proves sound but no valid signal is present, the fan assembly is likely defective.
- Engineer to inspect the differential pressure switch and hoses (where applicable) Gas Safe engineer
On models where 3A can indicate a pressure switch fault, the engineer will check the switch, its rubber connecting hoses for cracks or blockages, and the associated wiring. A faulty switch or collapsed hose can mimic a fan fault at the control board level.
- Replace the defective component — fan assembly, wiring harness, or pressure switch Gas Safe engineer
In the majority of cases the fan unit itself requires replacement. The engineer will source the correct part for the specific Buderus model, fit it, and verify the tacho signal is restored before clearing the fault and running the boiler through a full heat cycle.
Parts you may need
- Flue fan / blower assembly · from £150
- Fan wiring harness / plug · from £35
- Differential pressure switch · from £45
- Fan tacho sensor (where separate) · from £30
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 £225–£375, depending on the underlying cause.
Frequently asked questions
Is Buderus fault code 3A dangerous — can I keep using the boiler?
No, you should not attempt to override or repeatedly reset the boiler and continue using it. The fan is a critical safety component: it pushes combustion gases including carbon monoxide through the flue and out of your home. When code 3A appears the boiler locks out specifically to prevent gases building up inside the appliance or the property. Leave it locked out and arrange for a Gas Safe engineer to attend promptly.
Can I fix a Buderus 3A fault myself?
Beyond checking your gas supply is live and attempting one or two resets, there is nothing a homeowner should do. Removing the boiler casing breaks the safety room seal and must only be carried out by a Gas Safe registered engineer. Touching the fan, wiring, or any internal component without the correct qualifications is both unsafe and illegal under UK gas safety regulations.
How much does it cost to fix a Buderus 3A fault in the UK?
Most people with this fault pay between £225 and £375 all-in for parts and labour, with a straightforward fan replacement typically sitting in the middle of that range. Fan parts for Buderus boilers cost roughly £100–£200 depending on the model, and Gas Safe engineers typically charge £50–£80 per hour plus a call-out fee. Buderus is less common in the UK than brands like Worcester Bosch or Vaillant, so parts can occasionally take longer to source and may cost slightly more. In the unlikely event the PCB or UBA controller is also affected, costs could be considerably higher — worth discussing with your engineer before authorising work on an older boiler.
Why does the 3A code come back every time I reset the boiler?
If the fault code returns immediately or within a few minutes of resetting, the underlying component — most commonly the fan motor, a tacho magnet, or a damaged wiring connector — has physically failed and no amount of resetting will correct it. Stop resetting the boiler (more than two or three attempts can damage the ignition transformer) and book a Gas Safe engineer to carry out a proper inspection and replace the faulty part.