Vauxhall Zafira problem Code P0597?

I have the "spanner" light coming up on my 2006 1.8 zafira. The pedal test is showing code 0597, which I understand to be open or short thermostat heater control unit. It has been suggested to ne that it may be the coolant temperature heat sensor on the thermostat, but reading info on line tells me it could be the thermostat. The car doesn't appear to be overheating, and it otherwise seems to be driving normally. Heater ok and working properly. Could somebody tell me what they think the problem may be please?