BoilerFaultCodesGlow-worm F5 Fault Code
The Glow-worm F5 fault code means: your boiler has switched itself off for safety because it has reached an excessively high temperature, likely due to a circulation problem.. It appears on 11 Glow-worm models in our database.
Technical description: overheat lockout
What causes the Glow-worm F5 fault?
This fault occurs when the water inside the boiler gets too hot, causing a safety sensor to trip and shut the system down to prevent damage. The most common reasons are a pump failure that stops water from moving or a blockage within the pipework that restricts flow. In some cases, the sensor itself may have developed a fault and is incorrectly reporting a high temperature.
This issue is more frequent when the heating is first turned on after summer, as pump components can seize up during long periods of inactivity.
DIY fix possible
Check that your radiator valves are fully open and not turned down.Ensure there is enough water pressure in the system by checking the gauge.Try resetting the boiler by pressing the reset button to see if it clears the lockout.
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 F5 fault code
Click a model for the full fault code list and model-specific repair notes.
- 30ci Plus Repair: £120-350
- Ultracom 2 CXI Repair: £100-350
- Ultracom 2 HXI Repair: £100-350
- Ultracom 24cx 30cx Repair: £100-350
- Ultracom 35 Store Repair: £100-350
- Ultracom cxi Repair: £100-350
- Ultracom hxi Repair: £100-350
- Xtrafast 120 Repair: £120-280
- Xtrafast 96 Repair: £120-280
- Xtrafast 96 120 Repair: £120-280
- Xtramax HE Repair: £120-350
Source
Glow-worm F5 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