Sounds like OBD2 code P0128. I was getting this code and my coolant temps were all over depending on the outside temperature and driving conditions. I'd get an SES light every few days. I finally replaced the thermostat and it fixed my problem. When I looked it up on Google, one site said it could be these problems.
- Leaking or stuck open thermostat
- Low engine coolant level
- Insufficient warm up time
- Faulty engine coolant temperature sensor
- Engine coolant temperature sensor harness is open or shorted
- Engine coolant temperature sensor circuit poor electrical connection
Read more: http://engine-codes.com/p0128.html#ixzz3NzyufFYQ
