BoilerFaultCodesSime AL 43 Fault Code
Also written as AL43 — same fault.
The Sime AL 43 fault code means: your boiler has lost its connection with the smart thermostat or remote control system, meaning they can no longer talk to each other to manage your heating.. It appears on 14 Sime models in our database.
Technical description: Open Therm communication fault — check OpenTherm wiring/remote control connection
DIY fix possible
Check that your wall-mounted thermostat or remote control has power and fresh batteries.Inspect any visible wires between the thermostat and the boiler for signs of damage or loose connections.Turn the boiler power off at the fused spur switch, wait 30 seconds, and turn it back on to reset the communication link.
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Sime models that show the AL 43 fault code
Click a model for the full fault code list and model-specific repair notes.
- Brava One HE Repair: £90-180
- Brava Slim HE Regular Repair: £80-220
- Brava Slim HE System Repair: £80-180
- Dewy 30/130 HE FS Repair: £60-180
- Dewy 30/50 HE WM Repair: £90-180
- Giulia Combi 30 Repair: £100-250
- Halstead Trio Repair: £90-180
- Mia 30 Repair: £90-180
- Murelle One HE Repair: £80-180
- Murelle Pro HE 25 HO Repair: £80-180
- Murelle Pro HE Combi Repair: £90-180
- Murelle Pro HE MkII Combi Repair: £80-180
- Murelle Pro HE R MkII System Repair: £90-250
- Murelle Pro HE System R Repair: £70-160
Source
Sime AL 43 fault code information on this page is sourced and cross-checked against the official Sime service / installation manuals for each affected model.
- Sime Brava One HE manual (PDF)
- Sime Brava Slim HE System manual (PDF)
- Sime Dewy 30/130 HE FS manual (PDF)
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026