F75
FAULT CODE
Glow-worm
Pump / Pressure Sensor Fault
Medium Severity
DIY Check Possible
The boiler cannot detect a pressure change when the pump starts. Because Glow-worm is part of the Vaillant Group, this fault is identical to Vaillant F.75 — the most common fault across both brands.
Step-by-Step Fix Guide
-
Step 1
Check the system pressure gauge — it should read 1.0–1.5 bar
-
Step 2
Repressurise using the filling loop if below 1 bar
-
Step 3
Bleed all radiators to remove airlocks
-
Step 4
Reset the boiler
-
Step 5
If the fault returns, the pump or pressure sensor may need replacing — call a Gas Safe engineer
Gas Safe Reminder
Work on gas appliances must be carried out by a Gas Safe registered engineer. The steps above cover safe checks a homeowner can perform - any internal component replacement requires a qualified professional. If in doubt, always call an engineer.