Medium
AL 43 on Sime Murelle Pro HE System R

Your boiler is having trouble communicating with your external thermostat or smart home controller.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

Book a service

This fault should be looked at by a Gas Safe engineer within the next week or two. It may get worse if left.

What the manual says

Manufacturer manual

Open Therm communication fault — check OpenTherm wiring/remote control connection

From this model's manual
Verified against the Sime Murelle Pro He System R installation & service manual, page 33.
Plain-English guidance below. Severity, DIY steps, costs and parts are our interpretation to help you act, not text from the manual.

DIY fix possible

Check that your wall thermostat is powered on and has fresh batteries if neededEnsure the wires connecting your thermostat to the boiler are not loose or damagedRestart your internet router if you use a smart heating system like Nest, Tado, or Hive

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Related fault codes

AL 42View →AL 44View →

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