Check engine light on again?

about 6 months ago i replaced my radiator, hoses, and thermostat. my check engine light came on about a month ago and it said low coolant level, faulty thermostat, or sensor connection. last week i replaced thermostat again and replaced the temperature sensor. and got a tune up. i ped emissions. 3 days ago my check engine came on again. diagnostics showed the same problems. my gauge is on cold most of the time every now and then it goes to the middle but drops back down. what could the problem be? i have a 2003 hyundai elantra