F9
FAULT CODE
Glow-worm
Pressure Sensor / Low Pressure
Medium Severity
DIY Check Possible
The boiler pressure sensor is detecting an issue — either the system pressure is too low, or the sensor itself is faulty or blocked with sludge/magnetite deposits.
Step-by-Step Fix Guide
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Step 1
Check the pressure gauge — top up using the filling loop if below 1.0 bar
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Step 2
Set to 1.2–1.5 bar (do not exceed the red zone)
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Step 3
Reset the boiler
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Step 4
If F9 persists with correct pressure, the pressure sensor may be blocked — 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.