Common problems guide

The most common Glow-worm 38cxi problems

The faults most likely to send a Glow-worm 38cxi into lockout — with plain-English causes, what to check first, estimated repair costs and whether you need a Gas Safe engineer.

This model is discontinued. Parts can be harder to source and expensive. If repair costs are mounting, a new A-rated boiler may be the smarter choice.
13 documented codes
6 most common
3 DIY-safe checks
3 engineer needed

The 6 most common faults

01
F.22 High DIY-safe

Your boiler has stopped working because there is not enough water pressure in the system to operate safely.

What to try first

Locate the filling loop (usually two blue levers or a flexible silver hose) underneath the boiler.Open the taps slowly until you hear water entering the system.Watch the pressure gauge and close the taps once the needle reaches 1.5 bar.Reset the boiler using the flame symbol or power button.

Est. cost
£0-120
Parts
Filling loop, Expansion vessel, Pressure sensor
Full guide for F.22
02
F.77 High DIY-safe

Your boiler has detected a blockage in the drainage system or an issue with the flue, causing it to shut down to prevent water or exhaust gases from backing up.

What to try first

Check the white plastic pipe under the boiler for any obvious kinks or blockages.If the weather is freezing, check the external condensate pipe for ice and gently pour warm (not boiling) water over it to thaw.Inspect the external flue terminal for any visible obstructions like leaves or bird nests.

Est. cost
£90-180
Parts
Condensate trap, Condensate pump (if fitted), Flue pressure switch
Full guide for F.77
03
F9 High DIY-safe

Your boiler has detected that the water pressure inside your heating system is either too low or too high for it to operate safely.

What to try first

Check your boiler's pressure gauge located on the front panel.If below 1 bar, locate the filling loop (usually a silver braided hose under the boiler) and open the valves to let water in until the needle reaches 1.5 bar.Close the valves tightly and restart the boiler.

Est. cost
£80-150
Parts
Pressure sensor, Filling loop, Expansion vessel
Full guide for F9
04
F.10 High Engineer

Your boiler has detected an electrical fault with the internal sensor that monitors the temperature of the water flowing to your radiators, causing the system to shut down for safety.

Est. cost
£100-£180
Parts
Flow temperature sensor (NTC thermistor), Connecting wiring harness
Full guide for F.10
05
F.25 High Engineer

Your boiler has shut down because the exhaust fumes have become too hot, which is a safety measure to prevent damage to the internal components.

Est. cost
£120-350
Parts
Heat Exchanger (de-scaling), Flue Gas NTC Sensor, Burner Seals
Full guide for F.25
06
F.70 High Engineer

Your boiler's computer memory has been wiped or is mismatched, meaning it no longer recognizes which model it is supposed to be.

Est. cost
£100-£350
Parts
Printed Circuit Board (PCB), User Interface/Display PCB
Full guide for F.70
Parts most often replaced

on the Glow-worm 38cxi

Filling loopExpansion vesselPressure sensorCondensate trapCondensate pump (if fitted)Flue pressure switch

Based on parts cited in our fault code database. Your engineer will confirm what's actually needed after diagnosis.

Call a Gas Safe engineer if…

  • You can smell gas or see signs of a leak
  • The Glow-worm shows an Emergency or High severity code
  • The boiler keeps locking out after repeated resets
  • You've tried the DIY checks and the fault hasn't cleared
  • There's visible water leaking from the boiler
  • The flame is yellow or orange instead of blue
See the full 13-code list for the Glow-worm 38cxi

Related guides

Boiler losing pressure → No hot water → When to replace → Symptom checker →