Temp Gauge, AC Not Working

[phipps]

My daughter is having an intermittent issue in her 2007 Ion.

What is happening is is her temp gauge will drop and the AC stops working. At the same time she gets a CEL with a P0128 code and the cooling fan runs full bore.

When the gauge is working everything works fine.

The problem is intermittent. It wasn't working yesterday morning but when I was driving it yesterday evening it worked fine.

Any thoughts?

[phipps]

Turned out to be the Engine Coolant Temperature Sensor. Replaced it and all is well again.