Your boiler is producing more heat than your hot water tank can safely handle, causing the system to stop for protection.

The hot water tank is too small for this unit power (external hot water tank).

What causes this fault?

This fault occurs when the boiler generates heat faster than your hot water cylinder can absorb it, leading to a rapid temperature spike that triggers a safety shutdown. The most common reason is that the boiler's power output is too high for the size of your external water tank, or there is a restriction preventing the heat from transferring efficiently into the water.