ALARM 04 Medium

Sime ALARM 04 Fault Code

Also written as ALARM04 — same fault.

The Sime ALARM 04 fault code means: your boiler has a fault with the domestic hot water sensor, meaning it may still provide heating but cannot accurately control the temperature of your hot water.. It appears on 1 Sime model 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: DHW sensor (SS) fault — open or short circuit; boiler continues but without DHW temperature control; check/replace sensor
Typical repair cost £90-160 (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 ALARM 04: 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 ALARM 04 fault code

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

Source

Sime ALARM 04 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