Your boiler's water pressure sensor is providing an incorrect reading, which is preventing the system from confirming that there is enough water inside to operate safely.

Water pressure sensor signal in the wrong range (too low)

What causes this fault?

This fault occurs when the boiler's internal computer receives an electrical signal from the water pressure sensor that is outside its expected range. The most common reason is a build-up of sludge or debris blocking the sensor's opening, though it can also be caused by the sensor's internal electronics failing or a loose connection in the wiring.