Viessmann d6 Fault Code: Causes, Fixes & Repair Costs
What does the Viessmann d6 fault code mean?
The D6 fault code on Viessmann Vitodens boilers indicates that the burner control unit has detected a problem with the fan's operation. This could mean the fan is not reaching the correct RPM during a cycle, is not returning to a proper standstill between cycles, or a connection between the fan and the PCB is unstable. Because the fan must be confirmed as running before the PCB allows ignition to occur — it is the component responsible for pushing harmful combustion gases safely out through the flue — any doubt about its performance causes the boiler to lock out and display D6. Note that fault code meanings can vary slightly between Viessmann models, so always cross-reference with your specific boiler's manual if in doubt.
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 fan motor Common
Over time, the motor windings inside the fan can burn out, or the bearings can deteriorate and seize. When this happens, the fan either fails to reach the required speed or stops unexpectedly, triggering the D6 lockout. This is the single most frequent cause of this code.
- Loose or damaged wiring connections to the fan Common
The fan communicates its speed back to the PCB via a tachometer signal wire. If the plug-in connector at either end of that cable is loose, corroded, or damaged, the PCB receives no signal and assumes the fan is not running correctly, causing the lockout.
- Blocked or restricted flue Sometimes
A flue terminal that is partially obstructed by a bird nest, accumulated debris, or ice during cold weather can create enough back-pressure to prevent the fan from operating at the correct speed. The PCB detects the RPM discrepancy and shuts the boiler down.
- PCB failure Rare
If the control board itself develops a fault, it may misread the fan speed signal or fail to send the correct commands to the fan motor, producing a spurious D6 code even when the fan is physically fine. This is typically only confirmed once all other causes have been ruled out by an engineer.
How to fix it
- Check your gas supply is working DIY safe
Before anything else, confirm that other gas appliances in your home — such as a gas hob — are working normally. If there is no gas supply at all, contact your gas supplier rather than resetting the boiler.
- Inspect the external flue terminal for obvious blockages DIY safe
Go outside and look at the point where the flue exits the building. Check for any visible obstructions such as a bird nest, compacted debris, or ice build-up in very cold weather. If you can safely remove an obvious surface blockage without touching the flue pipe itself, do so — but do not dismantle any flue components.
- Reset the boiler once DIY safe
Press and hold the reset button on your Viessmann boiler for approximately three seconds, then release and wait around 60 seconds for the boiler to attempt a restart. Only do this once or twice at most. If the D6 code returns immediately or shortly after, further resets will not help and you should move on to calling an engineer.
- Fan inspection, testing, and replacement if necessary Gas Safe engineer
A Gas Safe registered engineer will remove the boiler casing, check the fan motor for signs of bearing wear or burnt windings, verify that the tachometer signal cable and plug-in connectors are secure, and test the fan's actual RPM against the manufacturer's specification. If the fan is faulty, it will be replaced with a compatible part.
- Wiring and connector diagnostics Gas Safe engineer
The engineer will inspect all electrical connections between the fan and the PCB for corrosion, fretting, or looseness. Even a slightly unseated connector can prevent the PCB from receiving a reliable speed signal, so all plug-in connections will be checked and re-seated or replaced as needed.
- PCB testing and replacement if required Gas Safe engineer
If the fan and all wiring are confirmed to be in good condition, the engineer will test the PCB for faults. A faulty control board that is misreading signals or failing to command the fan correctly will need to be replaced. This is typically a last-resort diagnosis after mechanical causes have been eliminated.
- Book a Gas Safe registered engineer if the fault persists Gas Safe engineer
If the boiler continues to display D6 after a single reset attempt, do not keep resetting it. Contact a Gas Safe registered engineer — ideally one who is Viessmann-trained — to carry out a proper diagnosis. Opening the boiler casing yourself is not safe and will likely invalidate your warranty.
Parts you may need
- Vitodens fan motor assembly · from £130
- Fan tachometer signal cable / wiring loom · from £25
- Boiler PCB (control board) · from £220
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–£375, depending on the underlying cause.
Frequently asked questions
Is the D6 code dangerous — can I keep using my Viessmann boiler?
You should not attempt to repeatedly override or reset a boiler displaying D6. The fan is responsible for expelling harmful combustion gases, including carbon monoxide, safely out of your home. If the boiler has locked out because it cannot confirm the fan is working correctly, it is doing so as a safety measure. Leave it locked out, attempt a single reset, and if the fault returns, call a Gas Safe engineer before using the boiler again.
My Viessmann boiler shows D6 — could it just be a wiring problem rather than a new fan?
Yes, absolutely. A loose or corroded plug-in connector on the fan's tachometer wire is a relatively straightforward fix compared to a full fan replacement, and it is worth an engineer checking the connections first before assuming the fan motor itself has failed. In some cases a connector re-seat or a replacement cable resolves the fault without the need for a new fan.
How much does it cost to fix a D6 fault on a Viessmann boiler?
For the most common repair — fan inspection and replacement — expect to pay in the region of £200–£375 including parts and labour. If the fix turns out to be a loose wiring connection or a minor adjustment, the cost may be closer to £100–£150 covering the call-out and labour. PCB replacement, if required, typically costs £180–£400 depending on the model, but this is less common. If your boiler is over ten years old and developing multiple faults, it may be worth discussing replacement with your engineer rather than investing in expensive repairs.
Will a D6 fault void my Viessmann warranty?
Not by itself — but how you respond to it might. Viessmann's extended warranties (up to 10 years on models such as the Vitodens 200-W) require the boiler to be installed and serviced by an approved Viessmann engineer. Attempting to open the casing yourself, or having unqualified work carried out, can invalidate that warranty. To protect your cover, always use a Gas Safe registered engineer who is ideally Viessmann-trained, and keep a record of annual services.