i've had my 2015 sonic LT for about a year now and just recently my check engine light came on. i immediately took it to get the codes checked and P0599 came up twice, I was told it was the thermostat. Then out of no where my water pump gave out, causing my coolant to constantly leak out at all times. I took it to a mechanic since my power train warranty has already expired and he fixed my water pump no problem and told me that i "should" get the thermometer fixed before 90,000 miles (I currently have 80,000) and that was that. One day later my CEL came back on and i re read the codes and it gave me P0599 again. I have read that the 1.8 liter Sonics thermostats go bad quite frequently and i'm so confused. This car has really given me so many issues in the 1 year that I have had it and It is so frustrating.