D.84 Low

Vaillant D.84 Fault Code

Also written as D84 — same fault.

The Vaillant D.84 fault code means: your boiler is simply letting you know that it is due for its routine annual service soon.. 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: Maintenance indicator – number of hours until the next maintenance

What causes the Vaillant D.84 fault?

This code is triggered because your boiler has a built-in timer or run-hour counter that tracks how long it has been since its last professional inspection. It is not a mechanical failure but rather a proactive reminder set by an engineer to ensure the appliance remains safe and efficient. Once the pre-set time limit is reached, the system displays this notification to nudge you to book your annual maintenance.

Typical repair cost £80-£120 (including labour)

Engineer required

This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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Vaillant models that show the D.84 fault code

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

Source

Vaillant D.84 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