Car running cold

My first guess would be that the sensor is bad.
Check the following;

Does the heater blow hot air after a reasonable amount of time to warm up?

Be carefule checking this. With engine warmed up, open the hood and grab the upper radiator hose. Does it have pressure? Check this agaqin with the vehicle turned off. It should be enough pressure to firmly resist you trying to squish it flat or collapse it.

What does the fluid in the overflow reserviour look like? Is it dirty?

I am assuming that you changed anti-freeze when you changed thermostat. Did you mix it 50/50 with water or buy a pre-mix?

Let me know the aforementioned and I will keep this post on my list and get back ASAP.

I agree with doctor cranknwrench, engine coolant temp sensor, sounds faulty

You can test a sensor, but it has to be removed from the block and then put into boiling water on your stove with a thermometer that goes to 180-230 Degrees Farenheit. You also need a multi-meter to test for continuity. Essentially at a certain temperature, ussually between 180-220 F, the switch should close or go to ground completing a circuit or triggering a step or rheostat. The difference is something I have not mentioned as cars often have a temp sensor for the computer to read and one for the temperature gauge on the dash< (if you have one). Sensors for temp can even be in the oil circuit as opposed to the coolant circuit. Generally the sensor that triggers the engine cooling fan is an oil temp sensor and will simply complete a circuit at a given temperature to turn on said fan and snensors for the computer and gauge are in the coolant. These are a little more complicated as they will have a variable resistance correspsonding with a temperature to send a certain voltge signal to computer and gauge.
You can get a Haynes Manual at your local auto store or if you want a really thourough manual, like a factory service manual, the Mitchell! Manual can be purchased through the site and is a great investment as you have manuals foir all your vehicles.
The given paramaeters for testing sensors can be found in either.
Personally, I start changing out sensors if they are not too expensive. Also, you might get an infra-red termeoter, (the point and shoot type) as they are very helpful. You can check temp aty the thermostat housing to make sure it us what it should be.

The fact that a CEL is getting triggered makes me think that the sensor in the coolant which is sending a signal to the ECU is bad.
There are other temp oriented sensors to also consider such as FAst IDLE and the IDLE AIR CONTROL VALVE. These will trigger problems at iddle or under load if they are bad.

Let me know if I can be of any more help.