ALL 21 Medium

Sime ALL 21 Fault Code

Also written as ALL21 — same fault.

The Sime ALL 21 fault code means: your boiler has lost the ability to accurately measure the temperature of the water being sent to your radiators, which may cause your heating system to operate inefficiently or stop working in specific zones.. It appears on 3 Sime models in our database.

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.

Technical description: DELIVERY PROBE FAILURE FIRST MIXED ZONE
Typical repair cost £120-220 (including labour)

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 for a qualified engineer in your area.

Gas Safe 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.

Sime models that show the ALL 21 fault code

Click a model for the full fault code list and model-specific repair notes.

Source

Sime ALL 21 fault code information on this page is sourced and cross-checked against the official Sime service / installation manuals for each affected model.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026