23 Medium

Sime 23 Fault Code

The Sime 23 fault code means: your boiler is having trouble communicating with the secondary heating circuit that controls a specific part of your home, such as an underfloor heating zone.. It appears on 2 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: ANOMALY SECOND MIXED ZONE
Typical repair cost £150-350 (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 23 fault code

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

Source

Sime 23 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