Your boiler has detected that water isn't moving through the unit as expected, likely due to a circulation issue or a sensor that is no longer properly attached.

Technical description

Flow/heat exchanger temperature sensor differential too great 1. Supply sensor is not assembled on the supply pipe. 2. Supply sensor does not make proper contact to the supply pipe. 3. System is not filled with water. 4. No water flow, because CH system is closed. 5. No water flow, because pump is not operating correctly.