BoilerFaultCodesGlow-worm S.03 Fault Code
Also written as S03 — same fault.
The Glow-worm S.03 fault code means: your boiler is failing to light the gas flame properly during its first stage of operation.. It appears on 12 Glow-worm models in our database.
Technical description: ignition
What causes the Glow-worm S.03 fault?
This fault occurs when the boiler's control board sends a signal to start the fire, but it fails to detect a stable flame during the initial ignition sequence. The most common reasons are a worn-out spark electrode that can no longer create a spark, or a debris-clogged ignition lead preventing the electrical current from reaching the burner.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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.03 fault code
Click a model for the full fault code list and model-specific repair notes.
- Flexicom 18sx Repair: £120-£350
- Flexicom cx Repair: £120-350
- Flexicom hx Repair: £120-£350
- Flexicom sx Repair: £120-£350
- Ultracom 2 CXI Repair: £100-350
- Ultracom 2 HXI Repair: £100-350
- Ultracom 2 SXI Repair: £120-300
- Ultracom 24cx 30cx Repair: £100-350
- Ultracom 35 Store Repair: £100-350
- Ultracom cxi Repair: £100-350
- Ultracom hxi Repair: £100-350
- Ultracom sxi Repair: £120-300
Source
Glow-worm S.03 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