High
F23 on Glow-worm Ultracom sxi

Your boiler has detected that the water inside the system is not circulating properly, meaning heat is not being moved from the boiler to your radiators or taps efficiently.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

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.

What the manual says

Manufacturer manual

Temperature Differential Fault (Flow/Return)

From this model's manual
Verified against the Glow Worm/Glow Worm Ultracom2 12sxi installation & service manual.
Plain-English guidance below. Severity, DIY steps, costs and parts are our interpretation to help you act, not text from the manual.

Engineer required

This is a high severity fault. Stop using the boiler and call a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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Related fault codes

F22View →F24View →

All 76 fault codes on the Ultracom sxi

Seeing a different code? Every code this model can display, in one place.

ErrF00F01F1F3F10F11F20F22F24F26F27F28F29F32F49F61F62F63F64F65F67F68F70F71F72F73F74F76F77F83F84F85F86P.01P.02P.03P.05P.06P.07S.00S.01S.02S.03S.04S.05S.06S.07S.08S.10S.11S.13S.14S.15S.16S.17S.20S.21S.23S.24S.25S.26S.27S.28S.30S.31S.32S.34S.39S.40S.41S.53S.54S.96S.98

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