Which sensor is faulty on boilermate 2000

Hi. Can anyone tell me how to narrow down the faulty sensor. When I press sw2 on the panel and get to DHW, this shows HEr which is faulty sensor. There are three sensors the DHW supply, the return water feed and the store tank. The DHW and return are both the same type, I intent on replacing both if these. But how can I be sure which sensor is faulty? Is there a procedure that Helios narrow it down?

The DHW sensors is the front one (nearest to you)
Behind it is the PHE sensor.
Located elsewhere pushed inside a pocket on the unit is the store sensor.
Be best to change PHE & DHW sensors.
Mike

Hi mike. Thanks for answering. Will try.