Vaillant F.90 Fault Code: Causes, Fixes & Repair Costs
What does the Vaillant F.90 fault code mean?
The F.90 code appears on Vaillant boilers when communication between the main PCB and either the display board or a connected storage tank module (such as an actoSTOR unit) has broken down. In plain terms, two electronic components that need to talk to each other have gone silent. The boiler detects this loss of communication and locks out as a safety precaution. On certain Vaillant models — particularly the 937 and similar combination-storage variants — F.90 specifically flags that the Shift Load Storage Tank Module link has been interrupted. On other models it more broadly indicates a PCB-to-display communication failure. Either way, the root cause sits firmly in the electronics, and the fix almost always requires a Gas Safe registered engineer.
General guidance only — not a substitute for professional advice. 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
- Incorrect or accidentally changed Device Specific Number (DSN) Common
Each Vaillant boiler stores a Device Specific Number that tells the PCB which modules and accessories are connected to it. If the DSN is inadvertently altered — for example by a user pressing buttons on the control panel — the boiler may start expecting a module that is not physically present, triggering F.90. This is one of the most common triggers and is often the first thing an engineer checks.
- PCB or display board replaced without resetting the DSN Common
When a PCB, display board, or both are swapped during a repair, the DSN settings must be re-entered via the engineer menu. If this step is skipped or performed incorrectly, the two new components cannot recognise each other and F.90 appears shortly after the boiler is powered up.
- Wiring or connector fault between PCB and display Sometimes
Loose, corroded, or damaged ribbon cables and plug connectors that carry the digital communication signal between the PCB and the display board can cause intermittent or permanent F.90 faults. Vibration over time or a recent service where components were disturbed are typical contributing factors.
- Hardware failure in the digital communication circuit Sometimes
A component-level failure within the PCB's communication circuitry — rather than a settings issue — can permanently break the link. This may also manifest as a Non-Volatile Memory (NVM) error, where the PCB loses its stored configuration data. Hardware faults like these require professional electronic diagnosis and usually result in PCB replacement.
How to fix it
- Reset the boiler once or twice DIY safe
On most Vaillant boilers the reset button is on the main display panel — look for the flame icon with a line through it. Press and hold it for approximately three seconds. If the boiler fires up and runs normally, monitor it over the next day or two. A one-off glitch can occasionally cause F.90. Do not reset more than two or three times; repeated resets without a lasting fix indicate a genuine fault that needs an engineer.
- Check that the gas supply to your home is on DIY safe
Confirm that other gas appliances (cooker, hob) are working and that your gas meter has not been isolated. If there is no gas supply at all, contact your gas supplier rather than a boiler engineer.
- Do not attempt to access the boiler's internal electronics Gas Safe engineer
Opening the boiler casing, adjusting engineer menu settings, or interfering with wiring or PCB connections must only be carried out by a Gas Safe registered engineer. Incorrect changes to the DSN or wiring can cause further faults or create a safety hazard.
- Have a Gas Safe engineer verify and correct the DSN setting Gas Safe engineer
An engineer will access the Vaillant engineer diagnostic menu to check whether the Device Specific Number matches the boiler's configuration and connected accessories. If the DSN has been altered or was not set correctly after a previous repair, restoring the correct value often clears F.90 without any parts being needed.
- Ask the engineer to inspect wiring and connectors between the PCB and display board Gas Safe engineer
The engineer should check ribbon cables and plug-in connectors for signs of looseness, corrosion, or damage. A reseated or replaced connector can resolve communication faults that look like expensive PCB failures.
- PCB diagnosis, NVM check, and replacement if required Gas Safe engineer
If the DSN and wiring are both in order, the engineer will carry out component-level diagnosis of the PCB's communication hardware and check for NVM errors. A faulty PCB will need to be replaced with a correctly coded Vaillant unit — the DSN must be programmed at the same time.
- Contact a Gas Safe registered engineer if the fault persists after your reset attempts Gas Safe engineer
Because F.90 is fundamentally an electronic fault, it will not resolve itself. If the boiler does not clear after a reset or two, book a qualified engineer promptly — especially in colder months when you need heating and hot water. You can use the Gas Safe Register website (gassaferegister.co.uk) to find a local engineer, or contact Vaillant directly on 0330 100 3143 to arrange a factory-trained technician visit.
Parts you may need
- Vaillant main PCB (model-specific) · from £280
- Vaillant display/user interface board · from £120
- PCB-to-display ribbon cable/connector harness · from £25
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–£500, depending on the underlying cause.
Frequently asked questions
Is the Vaillant F.90 fault dangerous?
F.90 is an electronic communication fault, not a gas leak or carbon monoxide issue. The boiler locks out as a precaution, which means it stops operating rather than running in a potentially unsafe state. You do not need to evacuate your home, but you should not attempt to open the boiler or override the lockout. Book a Gas Safe engineer to diagnose the cause properly.
Can I fix the F.90 code myself by resetting the boiler?
A single reset is worth trying — occasionally a momentary glitch causes the code and the boiler will run normally afterwards. However, if F.90 returns after one or two resets, the underlying cause (a wrong DSN setting, a wiring fault, or a failing PCB) will not go away on its own. All diagnostic and repair work beyond a basic reset must be done by a Gas Safe registered engineer.
How much does it typically cost to fix a Vaillant F.90 fault in the UK?
If the fix is simply correcting a DSN setting, you may pay little more than a call-out and diagnostic fee, typically £80–£150 depending on your region. If the PCB needs replacing — the more involved scenario — total costs including parts and labour generally fall in the £300–£500 range. Vaillant also offer a fixed-price repair package at around £399 (inc. VAT) that covers diagnosis, repair, and a gas safety check, which can represent good value if a PCB replacement is needed. In rare cases where both the PCB and display board require replacement simultaneously, costs can exceed £500–£600; if your boiler is over ten years old it may be worth getting a replacement quote at the same time.
Why did F.90 appear straight after my boiler was serviced or repaired?
This is a common scenario. If a PCB or display board was replaced during a recent repair and the engineer did not re-enter the Device Specific Number (DSN) correctly, the two components will fail to communicate and F.90 will appear once the boiler is powered up. Contact the engineer who carried out the work — they should return to correct the DSN setting, and this should not involve an additional charge if it was an error on their part.
Does my Vaillant warranty cover an F.90 fault?
If your boiler is within its manufacturer's warranty period and has been serviced annually by a Gas Safe registered engineer (a condition of most Vaillant warranties), the repair may be covered at no cost. Contact Vaillant directly on 0330 100 3143 or via the myVaillant web portal to check your warranty status before authorising any paid repair work.