D.90 Low

Vaillant D.90 Fault Code

Also written as D90 — same fault.

The Vaillant D.90 fault code means: your boiler is currently waiting for the digital control system to finish communicating, which usually means it is temporarily busy rather than broken.. It appears on 12 Vaillant models in our database.

Monitor it

This is a minor fault. Keep an eye on it and mention it at your next annual boiler service. Reset the boiler and see if it clears.

Technical description: Digital regulator status

What causes the Vaillant D.90 fault?

This code appears when the boiler's internal computer is trying to pair or exchange data with smart thermostats or external plug-in controls. It occurs because there is a brief delay or 'handshake' process happening between the boiler and the digital programmer. Most of the time, this is simply a temporary status message rather than a permanent hardware failure.

Typical repair cost £0-120 (including labour)

DIY fix possible

Check if any smart thermostats (like Hive, Nest, or vSMART) are currently updating their softwareTurn the boiler off at the main switch, wait 30 seconds, and turn it back onWait 10 minutes to see if the status clears automatically after communication is restored

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Vaillant models that show the D.90 fault code

Click a model for the full fault code list and model-specific repair notes.

Source

Vaillant D.90 fault code information on this page is sourced and cross-checked against the official Vaillant service / installation manuals for each affected model.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026