Vaillant F.90 fault codewhat it means and how to fix it
Communication with the actoSTOR hot water module — or on the 2022 CS/1-5 the internal cylinder — has been interrupted. A wiring fault on the link, or a setting left on after the module went.
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Get a Gas Safe registered engineer to look at your Vaillant F.90
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What F.90 means
The ecoTEC plus and exclusive can drive an actoSTOR, Vaillant's stratified hot water store that sits alongside the boiler and talks to it over a link Vaillant call the PEBus. F.90 is that conversation stopping. The plus table prints "fault: communication — communication with actoSTOR interrupted" and gives the engineer two steps: check the plug and the cable harness to the actoSTOR module, and, if the boiler is to run without an actoSTOR, set diagnostic code D.092 to 0 and refit the edge connectors — X31 into the boiler's board, X1 into the actoSTOR.
The exclusive prints the same in fewer words. That second step is the one that matters to most readers, because it explains the F.90 that appears when no module is present at all: the boiler has been told to expect one, either from the factory setting or from a store that has since been disconnected, and it reports the silence as a fault.
On the 2022 CS/1-5 the code is F.090, "communication with the internal cylinder has been interrupted", for the storage combi's built-in cylinder, with the harness and the PCB plug as the causes. F.90 is not a row on the pro, ecoFIT or ecoTEC sustain, none of which drive an actoSTOR.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| PEBus cable or plug fault | Vaillant's first check: the plug and the harness between boiler and module. A disturbed connector at either end stops the link. | Common |
| D.092 still set for a module that is not there | The boiler expects an actoSTOR because the setting says so. Vaillant's remedy is D.092 = 0 and the edge connectors refitted, an installer-level change. | Common |
| actoSTOR electronics fault | The module's own board has stopped answering. The exclusive manual describes an LED on the module that shows communication OK, not OK or no power. | Occasional |
| Internal cylinder link on a CS/1-5 storage combi | F.090's two causes: an interruption in the wiring harness, or the plug on the PCB loose or not plugged in. | Occasional |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£80–£150
Time on site
45 minutes
A plug or setting correction is a call-out of £80 to £120. A harness repair £100 to £150. A fault in the actoSTOR's own electronics is priced on the module.
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
Parts you may need
We may earn a commission if you buy a part through one of these links. It does not change what you pay, and it does not affect which parts we list.
Vaillant main PCB (electronics box)
£160–£340Authorised Vaillant parts distributor
Price checked 28 September 2026
Vaillant wiring harness (plug-and-cable set to a sensor or actuator)
£20–£65Authorised Vaillant parts distributor
Price checked 28 September 2026
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Related Vaillant fault codes
- F.49Get it checkedThe voltage on the eBUS, the two-wire link between the boiler and its Vaillant controls, has dropped too low. Usually a short or a wiring clash on that bus, not a boiler fault.Related: controls and communication
- conGet it checkedThe display has lost contact with the main board. "con" on a Vaillant is a communication fault between the user interface and the PCB in the electronics box, not a fault in the heating itself.Related: controls and communication
- S.39Get it checkedA protective wait, not a lockout: the "burner off" contact has opened — an underfloor thermostat or a condensate pump alarm wired to the boiler — and heating is blocked until it closes again.Related: controls and communication
- F.78Get it checkedA wiring-configuration code on UK combis: the external control's link box is connected but the hot water sensor input has not been bridged. Vaillant say the boiler itself has not malfunctioned.Related: controls and communication
- S.42Get it checkedA protective wait, not a lockout: an external device — a flue non-return flap through the VR40 module, or a condensate pump — is telling the boiler not to fire. Heating waits until it clears.Related: controls and communication
- F.86Get it checkedAn ecoFIT code: the underfloor heating contact is open. A floor-protection thermostat wired to the boiler has tripped, or its sensor is disconnected, and heating is blocked until it closes.Related: controls and communication
Own a Vaillant boiler? Save it and its codes come first next time.
Related guides
Technical review
Not reviewed by an engineer
No engineer has read this page. Severity follows what the manufacturer publishes, and we would rather tell you that than let a byline imply a sign-off.
Sources and how we verified this
We establish the facts from the sources below and then write the explanation ourselves. We do not copy manufacturer wording and we do not host their manuals.
- Manufacturer technical and service documentation
This page has not been reviewed by an engineer, and we would rather tell you that than imply otherwise. The severity rating follows what the manufacturer publishes: where they say isolate the appliance and call someone, so do we.