15 Medium

Firebird 15 Fault Code

The Firebird 15 fault code means: your boiler's secondary system is misreading the temperature because a connection to a heat sensor has become loose or damaged.. It appears on 1 Firebird 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: Water circuit 2 thermistor anomaly (THMwo2) - Loose, disconnected, broken or short-circuited connector
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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Firebird models that show the 15 fault code

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

Source

Firebird 15 fault code information on this page is sourced and cross-checked against the official Firebird service / installation manuals for each affected model.

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