I have a 2002 3.4 engine. The coolant light stays on but the radiator and resevoir are full. Could the thermostat be stuck or would this be a sensor issue? The temp runs normal.

 

Sensor issue. You can just unplug it and the light will stay off

Or you can try to clean it. There's a good chance the dex has turned a little cruddy on the sensor causing that error. You will lose some coolant taking the sensor out. I just have mine unplugged until my next flush.

 

Sounds like a bad sensor. A stuck thermostat would not cause the low coolant light to turn on.

I've had to replace my coolant sensor twice now. The first time, even the new sensor was bad right out of the box.

It's easy to swap out the sensor. Disconnect the electrical connector from the sensor, pry off the metal retaining clip (that's probably the hardest part), and pull / wiggle the sensor out. Be sure to place an old towel or shop rags underneath since coolant will start gushing out of the hole when you pull the old one out. If you're quick on the swap, you won't lose much coolant.