BoilerFaultCodesBiasi 4 + Fault Code
Also written as 4 — same fault.
The Biasi 4 + fault code means: your boiler has stopped working because the water pressure is either too low, too high, or the water isn't moving through the system correctly.. It appears on 2 Biasi models in our database.
Technical description: Pump circulation failure, insufficient system pressure or pressure above 3 bar, water pressure sensor not connected.
DIY fix possible
Check the pressure gauge on the front of the boiler; it should be between 1 and 1.5 bar.If the pressure is below 1 bar, use the filling loop underneath the boiler to carefully top it up.If the pressure is above 3 bar, bleed a radiator to release excess pressure.Restart the boiler once the pressure is corrected.
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Biasi models that show the 4 + fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Biasi 4 + fault code information on this page is sourced and cross-checked against the official Biasi service / installation manuals for each affected model.
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026