18 High

Firebird 18 Fault Code

The Firebird 18 fault code means: your boiler is struggling to read a temperature signal from an internal sensor, which is likely caused by a loose wire or a faulty connection.. It appears on 1 Firebird model 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: Auxiliary sensor 1 thermistor anomaly (THMaux1) - 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 18 fault code

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

Source

Firebird 18 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