Glow-worm Fault Code F.25 — Diagnose and Fix
Fault code F.25 on a Glow-worm boiler indicates NTC sensor. Flow/return thermistor out of range — cheap fix (£12–25). Here is the diagnostic order UK Gas Safe engineers follow.
Step-by-step method
1. Confirm the F.25 lockout
Power off at the fused spur for 60 seconds, restore and reset. If F.25 returns within 5 minutes, proceed. Intermittent F.25 points to a sensor or connection, not the PCB.
2. Check pressure, condensate, gas
System pressure 1.0–1.5 bar, condensate unfrozen, gas proven at another appliance — accounts for ~60% of lockouts.
3. Test what F.25 points at
Flow/return thermistor out of range — cheap fix (£12–25).
4. Reset and prove
Reinstate covers (interlocks matter), reset and observe full ignition. Flame current 1–8 µA DC.
5. Gas Safe callout
Gas-side work on a Glow-worm requires Gas Safe. Provide the fault code, GC number and serial so parts are pre-ordered.
Frequently asked questions
Reset Glow-worm F.25 myself?+
Hold reset 5 seconds. If it returns, fault is still present.
Repair cost?+
£120–£280 typical; PCBs £180–£320; sensors £60–£100.
Warranty cover?+
Yes if within warranty and annual services logged.