AL 56 High

Sime AL 56 Fault Code

Also written as AL56 — same fault.

The Sime AL 56 fault code means: your boiler has shut down because the temperature difference between the water going out and the water coming back is too high, suggesting the water isn't circulating properly.. It appears on 7 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: Lock for ΔT delivery/return over maximum limit — contact Technical Assistance
Typical repair cost £120-£350 (including labour)

DIY fix possible

Check that all radiator valves are fully openEnsure the boiler pressure gauge is between 1 and 1.5 barBleed all radiators to remove trapped airWait 15 minutes and press the Reset button

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Sime models that show the AL 56 fault code

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

Source

Sime AL 56 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