F.12 High

Glow-worm F.12 Fault Code

Also written as F12 — same fault.

The Glow-worm F.12 fault code means: your boiler has lost the ability to monitor the temperature of your hot water cylinder because of a faulty connection or a broken sensor.. It appears on 9 Glow-worm 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: Shortcircuit:Cylindertemperature sensor - NTCsensordefective,shortcircuitinthecableharness,cable/casing
Typical repair cost £90-160 (including labour)

Engineer required

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

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Glow-worm models that show the F.12 fault code

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

Source

Glow-worm F.12 fault code information on this page is sourced and cross-checked against the official Glow-worm service / installation manuals for each affected model.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 31 August 2026