Your boiler’s temperature sensors are giving conflicting readings, causing the system to shut down to prevent overheating.

Flow and/or return NTC fault | Flow and/or return NTC is defective (tolerances too big)

What causes this fault?

This fault occurs when the boiler's internal computer notices that the flow and return temperature sensors are reporting values that are too far apart or inconsistent with each other. The most common reason is that one of these sensors has drifted out of calibration or failed entirely, meaning the boiler can no longer safely monitor how hot the water is. To prevent the system from overheating, the boiler shuts down as a safety precaution.