Boiler shows A04, low water pressure, no heating

This repair may involve working with high voltage components or water connections. Always unplug the appliance before removing any panels.

If you are not confident in your ability to perform this repair safely, we strongly recommend contacting a professional technician.

Possible Causes

System leak, Recently bled radiators, Faulty pressure sensor, Undercharged expansion vessel

Safety first: Allow boiler to cool before topping up. Overfilling can cause discharge from the safety valve.

Check gauge: If below 0.8 bar, top up via the filling loop to 1.0–1.5 bar cold.
Bleed radiators: Remove air and re-pressurise as needed.
Inspect for leaks: Look for damp patches under the boiler and around radiators.
Engineer checks:
- Test pressure sensor.
- Check expansion vessel charge and integrity.

If pressure drops repeatedly, a leak or vessel fault must be located and repaired.

Medium 3/5

System Pressure Sensor

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