This code means your car hasn't reached 180* or so in a specified time. It means your thermostat is stuck open 99% of the time. You could also have some resistance in the ECT sensor or wiring. More temp = less resistance with thermistors so more resistance would make the PCM think it was a lower temperature.
The sensors measure poorly in air as well so a low coolant level may cause an air bubble to form near the sensor...but these aren't difficult motors to "burp".
I only have luck with Ford thermostats. I've had two Autozone stats stick open on me. I find the oil temp runs cooler than the coolant temp when this happens. Unless I really rev the motor the oil temp always was a little lower on the gauge.