Viessmann 91 Fault Code: Causes, Fixes & Repair Costs
What does the Viessmann 91 fault code mean?
Fault code 91 on Viessmann boilers signals that the burner control unit has detected the fan is still spinning (or producing an erroneous speed signal) when the boiler expects it to be completely at rest — typically just before ignition or after a shutdown cycle. Because the control board cannot confirm the fan is properly idle, it refuses to proceed and triggers a safety lockout. In some cases the fault is not the fan itself but an unstable or loose plug-in connection between the fan and the PCB, which creates a false signal that mimics a still-running fan. Either way, the boiler will not fire until the condition is resolved by a qualified 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
- Fan not reaching standstill between cycles Common
After a heating or hot-water call ends, the boiler expects the fan to spin down completely before the next ignition attempt. If the fan coasts for longer than the PCB allows — due to a worn bearing, a seized impeller, or a failing motor — the board sees it still running and locks out. This is the most frequently reported trigger for code 91.
- Loose or damaged wiring connector at the fan Common
The fan communicates its speed back to the PCB via a tachometer (Hall-effect) signal wire. A plug that has worked loose, corroded terminals, or a chafed cable can send erratic or constant signals that the PCB interprets as the fan never stopping, even if the fan hardware itself is fine.
- Incorrect fan RPM — speed outside expected range Sometimes
The burner management system continuously monitors fan speed during operation. If the fan motor is wearing out or the impeller is partially blocked, it may run at the wrong speed. On shutdown the spin-down profile also deviates, causing the PCB to conclude the fan is not properly at rest.
- Faulty fan motor Sometimes
A motor winding that is beginning to fail can produce intermittent or erratic speed feedback. The boiler may run normally for a while before the deteriorating motor triggers a lockout, making this fault appear random at first.
- PCB fault Rare
If the fan and all its wiring check out as healthy, the fault can lie in the PCB itself misreading or failing to process the fan standstill signal correctly. This is a less common cause but should be considered after other components have been eliminated.
How to fix it
- Attempt a boiler reset DIY safe
A single reset is worth trying in case the lockout was triggered by a momentary glitch. On most Vitodens models, turn the rotary dial briefly to the reset symbol then back to your normal setting. On some models you press 'Mode' and 'OK' together. Allow the boiler a full ignition cycle. If the fault clears and stays clear, monitor the boiler over the next day or two. If code 91 returns — especially quickly — do not keep resetting; move on to the next step.
- Check the boiler has adequate system pressure DIY safe
A low-pressure condition (below around 1 bar when cold) can compound faults. Check the pressure gauge on the boiler front. If it reads below 1 bar, top up via the filling loop until the gauge sits between 1 and 1.5 bar, then attempt another reset. If pressure is already correct, skip this step.
- Do not open the boiler casing or touch internal components Gas Safe engineer
Everything beyond this point is inside the appliance and involves electrical components directly connected to gas-combustion systems. Opening the casing yourself is unsafe, likely to void any remaining warranty, and may be illegal if it leads to work on gas-carrying parts. Leave all internal inspection to a Gas Safe registered engineer.
- Book a Gas Safe registered engineer to inspect the fan and wiring Gas Safe engineer
The engineer will remove the casing safely, run the boiler through a diagnostic cycle, and measure the fan's tachometer output against the manufacturer's expected values. They will check every plug-in connector on the fan loom for corrosion, back-out, or damage, and test the wiring continuity back to the PCB.
- Engineer to assess whether the fan motor or full fan assembly needs replacement Gas Safe engineer
If the motor windings are degraded, the engineer may be able to replace just the motor rather than the entire fan assembly, which is a lower-cost option. If the impeller or housing is also damaged, a full fan unit replacement will be required. Only genuine Viessmann-approved parts should be used to maintain boiler certification.
- Engineer to test or replace the PCB if the fan proves healthy Gas Safe engineer
Should the fan, its motor, and all wiring connections test as being within specification, the engineer will turn their attention to the PCB. A board that is misreading the standstill signal will need either a firmware update (where supported) or full PCB replacement.
Parts you may need
- Viessmann boiler fan / flue gas fan assembly · from £120
- Fan motor (replacement motor only, not full assembly) · from £55
- Fan tachometer / speed-sensor wiring loom · from £25
- 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 £200–£380, depending on the underlying cause.
Frequently asked questions
Can I reset a Viessmann fault code 91 myself and will it go away on its own?
You can safely attempt one reset — and occasionally a transient electrical glitch will clear after a reset. However, code 91 is a burner lockout caused by a physical or electrical fault with the fan, so it almost always returns if the underlying problem is not fixed. Resetting repeatedly without investigation will not resolve it and may mask a developing fault. If the code comes back after one reset, call a Gas Safe engineer.
How much does it cost to fix a Viessmann fault code 91 in the UK?
For most households, the repair falls between £200 and £380 including parts and labour. A straightforward wiring connector fix at the lower end, a fan motor swap in the middle, and a full fan assembly replacement at the higher end. If the PCB turns out to be the culprit, costs can rise to £400–£550 or more, but this is a less common outcome. Always get a written quote before authorising work.
Why does the fan need to be at a complete standstill before the boiler can fire?
Viessmann's burner management system uses a pre-ignition fan check to confirm the combustion chamber and flue are clear before introducing gas. If the fan is still spinning from a previous cycle — or the PCB thinks it is — the system cannot be certain that conditions are safe for ignition. Triggering a lockout in this situation is a deliberate safety feature, not a malfunction of the protection system itself.
My Viessmann boiler is 12 or more years old and keeps showing fault code 91 — should I repair or replace it?
If your boiler is over 12 years old and this is not the first significant repair, it is worth getting a replacement quote alongside a repair quote. Modern A-rated condensing boilers are considerably more efficient than older units, and the saving on gas bills can offset part of the replacement cost over time. A reputable engineer should be willing to give you an honest assessment of whether repair represents good value for your particular appliance.