51 Medium

Sime 51 Fault Code

The Sime 51 fault code means: your boiler has detected that the water inside the system is getting too cold and has activated its built-in safety feature to prevent the pipes from freezing.. It appears on 4 Sime 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: Antifreeze protection
Typical repair cost £100-200 (including labour)

Engineer required

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

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Sime models that show the 51 fault code

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

Source

Sime 51 fault code information on this page is sourced and cross-checked against the official Sime service / installation manuals for each affected model.

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