Viessmann 40 Fault Code: Causes, Fixes & Repair Costs
What does the Viessmann 40 fault code mean?
Fault code 40 on a Viessmann boiler indicates that the burner control unit has triggered a safety lockout because the fan was not at a complete standstill when expected, or because the plug-in connection to the fan is unstable. Viessmann boilers continuously monitor fan speed (RPM) throughout the heating cycle — including before ignition and after shutdown. If the fan is still spinning when it should have stopped, or if the control unit receives an erratic or missing speed signal, it locks the boiler out rather than risk unsafe combustion. This fault can appear on all Viessmann boiler models and always requires investigation by a Gas Safe registered engineer.
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 worn fan motor Common
The fan motor is the most common culprit behind fault code 40. As the motor ages or develops a fault, it may fail to reach a complete standstill between cycles, spin at the wrong RPM, or produce an erratic speed signal that the control unit rejects. A worn fan bearing is a particularly common cause of this behaviour.
- Loose or corroded plug-in connections to the fan Common
Viessmann's own guidance specifically flags an unstable plug-in connection as a direct trigger for this code. The electrical connectors linking the fan to the wiring harness can work loose over time, corrode, or suffer physical damage — causing the control unit to receive a faulty or intermittent RPM signal even if the fan itself is fine.
- Damaged fan wiring harness Sometimes
The cable running between the fan and the PCB can develop chafing, breaks, or heat-damaged insulation inside the boiler casing. A break in the circuit can produce the same erroneous fan-speed reading that triggers the lockout, even if the motor and connectors are otherwise intact.
- Faulty PCB (printed circuit board) Rare
The PCB interprets the fan speed signal and issues the lockout command. If the PCB itself is misreading the signal — due to a failed component, corrosion, or moisture ingress — it may raise fault code 40 even when the fan is performing correctly. PCB faults tend to come alongside other error codes or erratic boiler behaviour.
How to fix it
- Reset the boiler once or twice DIY safe
Press and hold the reset button on your Viessmann boiler for around 3 seconds, then release. Allow the boiler a few minutes to attempt a restart. If the fault clears and the boiler fires up normally, monitor it over the next day or so — a one-off lockout after an unusual power event can sometimes self-resolve. Do not attempt more than two or three resets; repeated lockouts mean the underlying fault needs professional diagnosis.
- Check that the gas supply is active DIY safe
Confirm that other gas appliances in your home (hob, oven) are working normally. If they are not, contact your gas supplier — there may be an interruption to your supply, which is unrelated to the fan fault. If your other appliances are fine, the gas supply to the boiler is not the issue.
- Do not attempt to access or adjust the fan Gas Safe engineer
The fan sits inside the sealed boiler casing, adjacent to the heat exchanger and flue system. Accessing it involves removing boiler panels and working close to gas components. This must only be done by a Gas Safe registered engineer. Do not remove any casing or attempt to free, clean, or test the fan yourself.
- Call a Gas Safe registered engineer to inspect the fan and its wiring Gas Safe engineer
An engineer will remove the boiler casing safely, visually inspect the fan motor for signs of wear or physical damage, and check every plug-in connector and the wiring harness for corrosion, looseness, or breaks. Many code 40 faults are resolved at this stage simply by reseating a connector or replacing a damaged cable — a relatively low-cost repair.
- Have the engineer test the fan motor and RPM signal Gas Safe engineer
If the wiring checks out, the engineer will run a functional test of the fan motor, verifying that it reaches the correct RPM on demand and drops to a full standstill at shutdown. If the motor is failing or the speed sensor is producing an out-of-range signal, the fan assembly will need to be replaced.
- Have the PCB inspected if the fan tests normal Gas Safe engineer
If the fan motor and all wiring prove sound, the engineer will assess the PCB. In some cases the board can be repaired; more often it requires replacement. If your boiler is still within its Viessmann warranty and the PCB failure is a manufacturing defect, this repair should be covered — contact Viessmann directly before authorising any chargeable PCB work.
Parts you may need
- Viessmann boiler fan assembly · from £130
- Fan wiring harness / plug-in connector set · from £35
- PCB (printed circuit 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 £120–£380, depending on the underlying cause.
Frequently asked questions
Is Viessmann fault code 40 dangerous?
The boiler has shut itself down as a precaution, so you are not in immediate danger — this is the safety system working correctly. However, you should not attempt to repeatedly reset the boiler or interfere with any internal components. Leave the boiler in lockout and arrange for a Gas Safe engineer to inspect it promptly, particularly during cold weather.
Can I fix fault code 40 myself?
The only homeowner-safe actions are a boiler reset (no more than two or three times) and checking that your gas supply is on. Everything else — accessing the fan, testing the wiring, or inspecting the PCB — must be carried out by a Gas Safe registered engineer. The fan sits inside the sealed boiler casing next to gas-carrying components, so this is not a DIY repair.
How much does it cost to fix a Viessmann fault code 40?
Most people with this fault pay somewhere between £120 and £380 all-in, depending on whether the fix is a wiring repair (lower end) or a full fan replacement (mid-range). PCB replacement, if required, tends to push costs higher and can reach £400–£500 or more depending on the specific board and model. If your Viessmann boiler is under warranty and the PCB has failed due to a manufacturing defect, contact Viessmann directly as this should be covered at no charge.
My Viessmann boiler keeps showing fault code 40 after resetting — what does that mean?
A fault that keeps returning after resets points to a genuine hardware problem rather than a one-off glitch. The most likely culprits are a worn fan motor that cannot reach a proper standstill, or a loose/corroded connector that reconnects briefly on reset before failing again. Repeated lockouts mean the underlying component needs to be repaired or replaced by a Gas Safe engineer — continued resetting will not fix the root cause and may mask a worsening fault.
Could an older Viessmann boiler be beyond economic repair if fault code 40 appears?
If your Viessmann boiler is ten years old or more and has been producing multiple fault codes over recent months, it is worth asking your engineer for an honest assessment of its overall condition. A fan replacement on an otherwise sound boiler is worthwhile; the same repair on a boiler with a corroding heat exchanger or an ageing PCB may not be cost-effective. A like-for-like Viessmann replacement typically starts at around £2,100 installed, but modern condensing boilers offer significantly better efficiency.