Medium
ALL 21 on Sime Murelle EV HE 25 Combi

Your boiler has detected an issue with a temperature sensor connected to an optional underfloor heating or multi-zone kit, causing that specific heating zone to stop working safely.

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 failure first mixed zone. When the mixed zone board is connected to the boiler, if the delivery probe becomes open or short circuit.

From this model's manual
Verified against the Sime/Sime Murelle Ev He Combi 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.

Engineer required

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

Find a Gas Safe engineer
Search the official register →
Boiler cover options

Avoid the next call-out bill

A cover plan would handle faults like ALL 21: no call-out fees, no surprise bills.

Approved partner
Domestic Appliance Guard
Unlimited call-outs, 24/7 helpline, no price-hike promise
Get a quote →

Affiliate links. We may earn a commission at no extra cost to you.

Related fault codes

ALL 20View →ALL 22View →ALL 23View →

All 22 fault codes on the Murelle EV HE 25 Combi

Seeing a different code? Every code this model can display, in one place.

ALL 02ALL 03ALL 04ALL 05ALL 06ALL 07ALL 08ALL 09ALL 14ALL 15ALL 20ALL 22ALL 23ALL 24ALL 25ALL 26ALL 27ALL 28ALL 29ERROR 10ERROR 13

Related guides you might find useful

Is boiler cover worth it?

Break-even maths on monthly cover.

Read →

When to replace your boiler

7 signs repair isn't worth it anymore.

Read →

Boiler service cost

What a UK service costs and what a proper one includes.

Read →