21 High

Heatline 21 Fault Code

The Heatline 21 fault code means: your boiler has lost the ability to accurately measure the temperature of the water flowing through the system, so it has shut down to prevent damage.. It appears on 3 Heatline models in our database.

Call an engineer soon

Book a Gas Safe engineer within 24 to 48 hours. Your boiler may be unsafe or could break down completely if left.

Technical description: Flow sensor fault (NTC)
Typical repair cost £120-220 (including labour)

Engineer required

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

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Heatline models that show the 21 fault code

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

Source

Heatline 21 fault code information on this page is sourced and cross-checked against the official Heatline service / installation manuals for each affected model.

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