AL 78 High

Sime AL 78 Fault Code

Also written as AL78 — same fault.

The Sime AL 78 fault code means: your boiler has detected an internal electrical fault with the gas control system, meaning it cannot safely manage the fuel flow and has shut down as a precaution.. It appears on 6 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: EV2 SGV upper limit error — check gas valve and PCB
Typical repair cost £180-450 (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 AL 78 fault code

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

Source

Sime AL 78 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