Your boiler has detected that the water temperature in one part of your heating system is higher than it should be, causing it to shut down for safety.

Overheating in zone 3

What causes this fault?

This fault occurs when the boiler's sensors detect an unsafe rise in water temperature specifically within the third heating zone of your system. The most common reasons are a motorised zone valve that has failed to open, a blockage in that section of pipework, or a pump that isn't circulating the water quickly enough to carry heat away from the heat exchanger.