[jdmccright]

This is a late post but I have the same problem with a too-cold coolant temperature.

The reason you were getting worse gas mileage is that the transmission's torque converter won't lock up until the engine temperature is high enough according to the ECM. So, if it is falsely reading too low it will not lock up in top gear, causing the engine/tranny to slip at highway speeds.

When my car's ('99 Intergra LS) temp gauge gets to about 1/3 above Cold I can drive at 70 mph and the engine will be turning around 2800 RPM. I know the TC locks up because if I try to accelerate, the RPMs rise directly with the car's speed.

But when the engine cools off to about 1/4 above Cold, I'll be going 70 mph @ 3000 RPM. Here when I accelerate, the RPMs rise and then the car's speed increases. This tells me the TC is not locked up.

At what temperature the TC is allowed to lock up I won't know until I test with my OBD scanner.

[UPDATE]
The TC is kicking out when the coolant temp drops below 168 degrees. I checked the temp sensor with an infrared thermometer pointed where it is screwed in and the OBD data and reading matched. So taking into effect surface cooling, the sensor is at least within reason. This leads me to believe that the thermostat is stuck open. If it were closing properly, it wouldn't let the coolant get so cold when I'm driving fast.