Your boiler's electronic brain has detected an internal safety error and has shut down for protection.

Safety relay failure (in boiler control). Switch OFF/ON - press RESET. Replace boiler control Part No. 5107535

What causes this fault?

This fault occurs when the boiler's internal safety checks detect a failure in the electronic components that manage gas flow and ignition. Essentially, a critical safety switch inside the control board has become stuck or damaged, preventing the boiler from firing up safely. Because the system can no longer verify that it is safe to operate, it shuts down to prevent any potential hazards.