B

Check the domestic hot water temperature sensor is connected and the harness is fitted securely to the sensor.
Check the resistance of the temperature sensor and replace if required.
Check the continuity of harness and replace if required.
When a faulty sensor/wiring issue is fixed the appliance will not need to be reset. The appliance will operate with a faulty sensor with fault 1021 displayed, estimating the DHW temperature from the supply sensor but with a reduced outlet temperature.
If the fault was the sensor falling off the pipe then a power cycle of the appliance will be needed to reset the fault.