High
2.5 on Worcester Bosch ZB 7 27 HE

Your boiler has detected that the water inside is getting too hot and has temporarily switched itself off to prevent damage.

Call an engineer soon

Book a Gas Safe engineer within 24 to 48 hours. Your boiler may be unsafe or could break down completely if left.

What the manual says

Manufacturer manual

Service Function: Max. CH flow temperature

From this model's manual
Verified against the Worcester Bosch Greenstar 25i Zwb 7 25 installation & service manual.
Plain-English guidance below. Severity, DIY steps, costs and parts are our interpretation to help you act, not text from the manual.

What causes this fault?

This fault occurs when the water inside the boiler gets too hot too quickly, causing the system to shut down for safety. The most common reasons are a lack of water flow caused by a sticking pump or a sensor that has become faulty and is giving a false temperature reading. It can also happen if air is trapped in the heat exchanger or if the system is partially blocked with debris.

This fault is more common in autumn when homeowners first turn their heating back on after summer, as pump components can seize up during long periods of inactivity.

Engineer required

This is a high severity fault. Stop using the boiler and call a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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