Medium
ALL 21 on Sime Murelle HE Regular

Your boiler is struggling to accurately measure the water temperature for your central heating because a sensor has stopped sending signals.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

Book a service

This fault should be looked at by a Gas Safe engineer within the next week or two. It may get worse if left.

What the manual says

Manufacturer manual

dEliVEry proBE BrEaKdown Er- ror first mixEd zonE

From this model's manual
Verified against the Sime Murelle He Regular installation & service manual, page 41.
Plain-English guidance below. Severity, DIY steps, costs and parts are our interpretation to help you act, not text from the manual.

Engineer required

This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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Related fault codes

AL 20View →AL 22View →

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