Your boiler is struggling to control its power level correctly, which may lead to it shutting down to prevent damage.

Actual burner modulation: Actual burner modulation.

What causes this fault?

This fault occurs when the boiler's internal computer is unable to accurately communicate with the gas valve to throttle the flame up or down. It typically happens because the electronic signals sent to regulate the fuel flow are being interrupted or misinterpreted by a failing control board. Without the ability to precisely control its power level, the boiler shuts down as a safety precaution.