12 Medium

Alpha 12 Fault Code

The Alpha 12 fault code means: your boiler has lost connection with the sensor that monitors the temperature of your hot water cylinder, meaning it may not know when to heat your water.. It appears on 24 Alpha models 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: Cylinder sensor fault (FlowSmart only)

What causes the Alpha 12 fault?

This fault occurs when the main control board can no longer detect an electrical signal from the temperature probe attached to your hot water tank. It is usually caused by the sensor internal components failing over time or a loose connection in the wiring that links the cylinder to the boiler.

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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Alpha models that show the 12 fault code

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

Source

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

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