BoilerFaultCodesSime AL 56 Fault Code
Also written as AL56 — same fault.
The Sime AL 56 fault code means: your boiler has switched itself off because the temperature difference between the water leaving and returning is too high, which usually means the water isn't flowing through the system correctly.. It appears on 6 Sime models in our database.
Technical description: Lock for ΔT delivery/return over maximum limit — contact Technical Assistance
DIY fix possible
Check that all radiator valves are fully openEnsure the boiler pressure gauge is between 1.0 and 1.5 barBleed all radiators to remove trapped air that might be blocking flowPress the Reset button once the system has cooled down
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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 31 August 2026