BoilerFaultCodesGlow-worm S.98 Fault Code
Also written as S98 — same fault.
The Glow-worm S.98 fault code means: your boiler is temporarily pausing to check its temperature sensors before heating your home, which may cause a delay in getting hot water or heating.. It appears on 12 Glow-worm models in our database.
Technical description: Flow / Return - sensor check is running, demand (DHW or heating) is blocked
What causes the Glow-worm S.98 fault?
This fault occurs when the boiler detects an unusual difference in readings between its two temperature sensors, prompting it to pulse the pump and compare the figures. The most common reason is that the sensors have become uncalibrated or the water isn't moving through the heat exchanger quickly enough for the boiler to trust the data. If the readings don't align during this check, the boiler will temporarily prevent itself from firing to avoid overheating.
DIY fix possible
Check that all radiator valves are fully openEnsure the system pressure is between 1.0 and 1.5 barWait up to 15 minutes to see if the code clears automatically
Find a Gas Safe engineer
Search the official register for a qualified engineer in your area.
Gas Safe Register →Glow-worm models that show the S.98 fault code
Click a model for the full fault code list and model-specific repair notes.
- Flexicom 18sx Repair: £120-220
- Flexicom cx Repair: £90-180
- Flexicom hx Repair: £120-220
- Flexicom sx Repair: £120-220
- Ultracom 2 CXI Repair: £90-180
- Ultracom 2 HXI Repair: £90-180
- Ultracom 2 SXI Repair: £90-180
- Ultracom 24cx 30cx Repair: £90-180
- Ultracom 35 Store Repair: £90-180
- Ultracom cxi Repair: £90-180
- Ultracom hxi Repair: £90-180
- Ultracom sxi Repair: £90-180
Source
Glow-worm S.98 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