S.53 Medium

Glow-worm S.53 Fault Code

Also written as S53 — same fault.

The Glow-worm S.53 fault code means: your boiler has temporarily paused because the water temperature is rising too quickly or unevenly, suggesting a circulation issue or a sensor fault.. It appears on 12 Glow-worm 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: waiting time: temperature difference flow - return sensor too high (Check thermistor fit)

What causes the Glow-worm S.53 fault?

This fault occurs when the boiler detects that water is heating up far too quickly or that there is a significant temperature gap between the heat going out and coming back. It is physically caused by poor water flow through the unit, often due to a blockage, a failing pump, or a sensor that is no longer reading the temperature accurately. Essentially, the heat cannot be moved away from the burner fast enough, so the system pauses to prevent overheating.

This issue is more frequent in autumn when heating is first turned on after summer, as pump components can seize or sludge can settle during periods of inactivity.

Typical repair cost £90-180 (including labour)

DIY fix possible

Check that all radiator valves are fully openEnsure the boiler pressure is between 1.0 and 1.5 bar on the displayBleed all radiators to remove trapped air that might be blocking water flow

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Glow-worm models that show the S.53 fault code

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

Source

Glow-worm S.53 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 27 April 2026