Boiler indicates a circulation fault; flow temperature rises too quickly or there is insufficient flow.

Safety: Isolate power and allow the boiler to cool before touching the pump or valves.

Steps:

• Check valves: Ensure the boiler flow and return isolation valves are fully open.
• Bleed air: Bleed radiators and any automatic air vents near the boiler.
• Check pump: With power restored and a heat demand present, listen for the pump running. Some Bosch pumps have a manual bleed/vent screw; with power off, carefully loosen it to check for rotation and free the shaft if stuck (have a cloth ready for small water drips).
• Inspect filters: Clean any magnetic or mesh filters on the return line.

If the pump does not run or is noisy and hot, it likely needs replacement by a heating engineer.