Viessmann 90 Fault Code: Causes, Fixes & Repair Costs
What does the Viessmann 90 fault code mean?
Fault code 90 on a Viessmann boiler signals that the burner control unit has entered a lockout or emergency mode because the combustion fan has failed to come to a complete stop when it should have, or is running at an unexpected speed. The boiler's control board continuously monitors fan RPM — if those readings don't match what's expected during the standstill phase, the boiler shuts down as a safety precaution to prevent unsafe combustion conditions. A faulty or loose plug-in connection between the fan and the control board can produce the same incorrect signal and trigger the same lockout. This code applies broadly across the Viessmann range and always warrants 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
- Worn or failing fan motor Common
The combustion fan runs thousands of hours each year and its motor can degrade over time. A worn fan may overshoot or undershoot the required RPM, or fail to spin down to a full stop when the burner switches off. The control board detects this discrepancy and raises fault code 90. This is the single most common root cause of this code.
- Loose or damaged plug-in wiring connections Common
The fan communicates its speed back to the PCB via a dedicated wiring harness and plug connector. If that connector is corroded, has worked loose through vibration, or a wire is chafed, the PCB receives a garbled or absent signal — which it interprets the same way it would a genuine fan fault. Sometimes reseating the connector is all that's needed, but the connector or cable itself may need replacing.
- Water ingress or moisture damage to the fan assembly Sometimes
Condensate or a leaking pump can allow moisture to reach the fan housing or its electrical connections. Water damage can cause intermittent fan speed errors and corrode the plug contacts. An engineer will look for signs of dampness or rust around the fan area when investigating this code.
- PCB fault or communication error Sometimes
The printed circuit board is responsible for sending speed commands to the fan and reading the feedback signal. If the PCB itself is malfunctioning — due to age, a power surge, or component failure — it may misinterpret a healthy fan as running incorrectly, or fail to issue the stop command at all. A PCB fault is usually only diagnosed once the fan and wiring have been checked and cleared.
- Blocked or restricted flue system Rare
A partial blockage in the flue or air intake can cause back-pressure that forces the fan to work harder and spin differently to expected parameters. While this is a less direct cause, it can produce unusual fan behaviour that the control board flags under fault code 90.
How to fix it
- Reset the boiler once using the reset button DIY safe
Press and hold the reset button (refer to your boiler's user guide for its exact location) until the boiler attempts to restart. Wait at least 60 seconds to see whether it fires up successfully or locks out again. If it locks out a second time, do not keep resetting — repeated resets without fixing the underlying fault can cause further wear. One reset is the homeowner's limit for this code.
- Check that your gas supply is working DIY safe
Make sure other gas appliances in the home (hob, gas fire) are working normally. If they're not, the issue may be with your gas supply rather than the boiler — contact your gas supplier.
- Inspect fan wiring and plug-in connections Gas Safe engineer
A Gas Safe engineer will check the connector between the fan harness and the PCB for looseness, corrosion, or pin damage. This is often a quick fix — reseating or replacing a damaged connector can clear the fault without further parts costs.
- Test and inspect the fan motor Gas Safe engineer
The engineer will run the fan through its start and stop cycle while monitoring the RPM feedback signal. A fan that runs erratically, fails to stop cleanly, or shows signs of mechanical wear or water damage will be replaced. Viessmann fans are model-specific, so the engineer will source the correct part.
- Check for moisture or water damage around the fan assembly Gas Safe engineer
If condensate tracking or a leaking pump is suspected, the engineer will trace the source of moisture and repair it before fitting a new fan — otherwise the replacement part could be damaged in the same way.
- Test the PCB if fan and connections are sound Gas Safe engineer
Should the fan and all wiring check out correctly, the engineer will test the PCB's output signals. A faulty board will need to be replaced with the correct Viessmann part. If your boiler is still within its manufacturer's warranty period, contact Viessmann directly — PCB replacement is typically covered at no cost to you.
- Call a Gas Safe registered engineer Gas Safe engineer
Fault code 90 always requires a qualified engineer to diagnose and resolve safely. Book a Gas Safe registered heating engineer and quote fault code 90 so they can bring appropriate parts. Do not attempt to open the boiler casing or disturb any internal components yourself — doing so can void your warranty and creates a safety risk.
Parts you may need
- Viessmann combustion fan (model-specific) · from £120
- Fan wiring harness and plug connector · from £25
- Viessmann PCB (control board, model-specific) · from £200
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 £250–£420, depending on the underlying cause.
Frequently asked questions
Can I fix Viessmann fault code 90 myself?
The only homeowner-safe step is a single reset attempt. Everything beyond that — inspecting wiring, testing the fan, or touching any internal component — must be done by a Gas Safe registered engineer. Viessmann boilers require specific diagnostic tools to read fan RPM data accurately, and disturbing internal wiring without the right knowledge risks both personal safety and invalidating your warranty.
How much does it cost to fix fault code 90 on a Viessmann boiler?
Most people with this fault pay between £250 and £420 in total, covering call-out, labour, and parts. A fan replacement typically accounts for most of that. If the PCB is the culprit rather than the fan, costs can rise to £400–£550 depending on your boiler model, as PCBs are more expensive components. If your boiler is still under a Viessmann manufacturer's warranty, a faulty PCB is usually covered at no charge — check your documentation and contact Viessmann before booking an independent engineer.
Why does fault code 90 keep coming back after a reset?
A successful reset tells the boiler to try again, but if the underlying fault — a worn fan, a loose connector, or a failing PCB — hasn't been fixed, the control board will detect the same problem and lock out again, often within minutes. Repeatedly resetting without investigation doesn't resolve anything and can accelerate wear on the fan motor. If the boiler locks out more than once, stop resetting and call an engineer.
My Viessmann boiler is 10–12 years old and showing code 90 — is it worth repairing?
At that age the boiler is approaching the end of its typical service life and may be coming to the end of its extended warranty period. A fan replacement is a reasonable repair on a boiler in otherwise good condition. However, if the PCB is also failing, or if you've had several faults in recent years, it's worth getting an engineer's honest opinion on whether repair or replacement offers better value. Modern A-rated condensing boilers are significantly more efficient than older units and can lower your energy bills.