Hello, I'm Jason. I will do my best to answer your question quickly and accurately using the information you have provided.
This code comes on when the computer senses that coolant temperature is too low, or the engine doesn't come up to temperature quickly enough after a cold start
There are usually 1-3 reasons for this
1) faulty coolant temperature sensor. This part is installed in the plastic coolant outlet flange at the rear of the cylinder head. It's a 4-pin sensor, with 2 pins sending information to the instrument cluster, and 2 to the engine ECU
2) faulty thermostat. When these fail, they fail in the open position, which will keep temperatures too low
3) cracked water pump impeller - if the water pump is original, and/or the timing belt has more than 70,000 miles on it, it's time for a timing belt and water pump replacement, and at the same time, I strongly recommend replacing the thermostat, and coolant temperature sensor as well