Your boiler is reporting a minor sensor issue related to your solar hot water system, but it is not a breakdown of the central heating itself.

Cylinder temperature bottom: Is only displayed, if a solar module is connected.

What causes this fault?

This fault occurs when the boiler loses communication with the sensor located at the bottom of your solar hot water cylinder. The most common reason is a faulty temperature sensor (NTC) or a break in the wiring connecting the solar control module to the tank. While your central heating will continue to work, the system can no longer monitor how much free heat is being provided by your solar panels.