2002 Mercury Marquis

P0125= Thermostat monitor, it's designed to verify propery tstat operation.
Insufficient temp for closed loop test, "closed loop" mean warm engine when input
from 02 sensors and all sensors is active. This monitor watches the engine
coolant temp sensor, the cylinder head temp sensor and the intake air temp
sensor, so either one of these sensors in no good, or the thermostat may be
opening too early. Engine coolant temp is lower than expected during the test,
which only occurs once per drive cycle. Cheapest way out is to change tstat
first, and see if code resets or not. Unless you know how to test these
sensors. With a cold engine, cooled overnight if your code reader can read
sensor values, hook it up in the AM, turn key on, do not start car, take temp
readings of the ect and iat, they should be within 18 degrees of each other. The
closer the better.