21 Medium

Vaillant 21 Fault Code

The Vaillant 21 fault code means: your boiler's internal control system is having trouble communicating with an optional external accessory, such as an extra pump or a remote timer device.. It appears on 1 Vaillant model in our database.

Book a service

This fault should be looked at by a Gas Safe engineer within the next week or two. It may get worse if left.

Technical description: Additional relay (select via D.026)
Typical repair cost £120-250 (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 21 fault code

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

Source

Vaillant 21 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