ALL 18 High

Sime ALL 18 Fault Code

Also written as ALL18 — same fault.

The Sime ALL 18 fault code means: your boiler has detected that water produced during the heating process is not draining away properly and is backing up inside the system.. It appears on 3 Sime models in our database.

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.

Technical description: Condensate level fault
Typical repair cost £120-200 (including labour)

Engineer required

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

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Sime models that show the ALL 18 fault code

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

Source

Sime ALL 18 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