d.16 Low

Heatline d.16 Fault Code

Also written as D16 — same fault.

The Heatline d.16 fault code means: your boiler is currently receiving a signal from your thermostat that is telling it to either turn on or off, and this code is simply showing you the active status of that connection.. It appears on 7 Heatline 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: Current state of the 24V room thermostat

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 d.16 fault code

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

Source

Heatline d.16 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