D.70 Low

Vaillant D.70 Fault Code

Also written as D70 — same fault.

The Vaillant D.70 fault code means: your boiler is currently stuck in a diagnostic test mode that overrides its normal settings, which usually happens during a service or repair.. It appears on 17 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: Set diverter valve position

What causes the Vaillant D.70 fault?

This code appears because the boiler has been manually instructed to hold its internal diverter valve in a specific position for testing purposes. This usually happens during a professional service when a technician needs to isolate the heating or hot water circuits to check for leaks or performance issues. If the code remains on the screen, the boiler has simply not been exited out of this specific diagnostic menu.

Typical repair cost £0-90 (including labour)

DIY fix possible

Locate the display panel on the front of the boilerPress the 'Reset' button (marked with a flame or cross symbol) for one secondWait for the boiler to restart and check if the 'D.70' code has disappeared

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Vaillant models that show the D.70 fault code

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

Source

Vaillant D.70 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