2002 GTP OBD code?

I have a 02 GTP and the check engine light came on and we put it on a handheld computer and it gave the code P0128. Anyone know what this is? Thanks.

The coolant temperature is below the thermostat regulating temperature. Typically a bad thermostat or improper rated thermostat. If you are stock best to use the OEM 195 degree or nothing less than 180 degrees.