Hello

Astra J 2010 1.6 has servivce light on. Can be reseted by Menu button but comes back after 10 minutes. Code reader gives code 597 - related obviously to thermostat. Engine temp does not come higher than 75C, normally about 90C.  Both upper and lower cooler hoses are warm but not hot. Heater and ac works normally (blowing warm air). Runs smoothly all seems to be OK. Please help whether this is typical failure and where to start to fix it. Does this engine have electrical controlled/heate