Viessmann 1A Fault Code: Causes, Fixes & Repair Costs
What does the Viessmann 1A fault code mean?
Fault code 1A on a Viessmann boiler indicates that the boiler's control board has detected the fan is still spinning — or spinning at an unexpected speed — when it should be completely idle, or that the electrical connection between the fan and the control board is unreliable. As a safety response, the boiler locks itself out to prevent operation until the problem is resolved. Because the flue fan is responsible for safely expelling combustion gases from your home, the boiler will not restart until the fan system is confirmed to be working correctly. The exact presentation of this fault can vary slightly between Viessmann models, so professional diagnosis is always recommended.
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 seized fan motor Common
The most frequent cause of a 1A code is a fan motor that has failed mechanically. Over time, the motor bearings can deteriorate or the windings can burn out, preventing the fan from spinning down to a proper idle. The boiler detects the incorrect RPM reading and immediately locks out as a safety precaution.
- Loose or damaged plug-in connection Common
The fan connects to the PCB via a wiring harness with plug-in connectors. Vibration, heat cycling, and age can cause these connectors to work loose, corrode, or suffer damaged terminals. The control board then receives inconsistent or absent signals from the fan, which it interprets as the fan not being at standstill, triggering the 1A lockout even if the fan itself is physically fine.
- Wiring harness damage Sometimes
The cabling between the fan and the PCB can develop faults — chafed insulation, broken cores, or heat-damaged sections — that disrupt the signal. This differs from a loose connector in that the wiring itself needs repair or replacement rather than just reseating.
- PCB fault Rare
If the fan and all its wiring check out as healthy, the printed circuit board may be sending incorrect control signals or misreading the fan speed feedback. A faulty PCB can cause the 1A code to appear without any genuine issue with the fan itself, though this is less common than a mechanical fan failure.
How to fix it
- Attempt a single boiler reset DIY safe
Locate the reset button on your Viessmann boiler (usually a flame symbol button or a dedicated reset function in the menu, depending on your model). Press and hold as instructed in your user manual. If the boiler fires up and runs normally, monitor it closely. If the 1A code returns within a short time, do not keep resetting — a recurring fault needs professional attention, not repeated resets.
- Check your gas supply is active DIY safe
Before assuming the boiler is at fault, confirm that other gas appliances in your home (cooker, gas fire) are working normally. If there is no gas at other appliances, contact your gas supplier rather than an engineer.
- Inspect the fan and its electrical connections Gas Safe engineer
A Gas Safe engineer will remove the boiler casing and visually inspect the fan assembly. They will check for signs of seized bearings, burnt windings, or physical damage to the fan blades. At the same time, they will examine the plug-in connectors and wiring harness for corrosion, looseness, or damage, reseating or replacing connectors as needed.
- Measure fan speed and verify RPM behaviour Gas Safe engineer
Using diagnostic tools, the engineer will run the boiler through its startup and shutdown sequence to confirm whether the fan is reaching the correct speed and — critically — spinning down to a complete idle as expected. An incorrect idle reading is the core trigger for the 1A code.
- Replace the fan motor if required Gas Safe engineer
If the fan motor has seized, the bearings have worn out, or the windings have failed, the fan unit will need to be replaced with a manufacturer-approved part. This is one of the more common outcomes for a 1A fault and restores safe flue gas extraction.
- Test and replace the PCB if all other components are healthy Gas Safe engineer
Should the fan, connectors, and wiring all prove to be in good condition, the engineer will assess whether the PCB is generating incorrect signals. If the PCB is confirmed faulty, it will need to be replaced — this is a less common but more costly outcome.
- Call a Gas Safe registered engineer Gas Safe engineer
Fault code 1A is not a homeowner-fixable fault. Any work beyond a single reset requires a Gas Safe registered engineer with Viessmann experience. You can verify an engineer's registration at gassaferegister.co.uk. If your boiler is still under its Viessmann warranty, contact Viessmann directly or use a Viessmann-trained installer to avoid invalidating your cover.
Parts you may need
- Viessmann flue fan / combustion air fan assembly · from £120
- Fan wiring harness / connector set · from £25
- Viessmann PCB (main control 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 £150–£400, depending on the underlying cause.
Frequently asked questions
Can I fix a Viessmann 1A fault myself?
No. A single boiler reset is the only safe homeowner action. The 1A fault relates to the flue fan — the component responsible for safely removing combustion gases from your home. Incorrect operation of this part poses a carbon monoxide risk, so any inspection, repair, or replacement must be carried out by a Gas Safe registered engineer.
How much does it cost to fix a Viessmann 1A fault in the UK?
Most people with a 1A fault pay somewhere between £150 and £400. A wiring or connector issue caught during a standard call-out will sit at the lower end. A full fan motor replacement typically runs £225–£350 including labour. If the PCB turns out to be the cause, costs can rise to £500 or more once parts and labour are combined, though PCB faults are the least common explanation for this code.
Why does the 1A code keep coming back after I reset the boiler?
A reset only clears the lockout temporarily — it does not fix the underlying fault. If the fan motor is worn, a connector is loose, or the PCB is misbehaving, the same conditions will trigger the code again within minutes or hours. Repeated resets can mask a worsening fault, so a recurring 1A code should be investigated by an engineer promptly rather than managed with repeated resets.
Is a Viessmann 1A fan fault covered by warranty?
It may be, if your boiler is within its warranty period and has been serviced annually by a qualified engineer as required by Viessmann's warranty conditions. If annual services have been missed, Viessmann may decline the claim. Check your warranty documentation and contact Viessmann or a Viessmann-trained installer to make a warranty enquiry before commissioning an independent repair.