code p0125 on 1999 Nissan Sentra

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i have a code p0125 according to the code could be a low coolant level,faulty thermostat or ect circuit fault initialy i have a little bit of low fluid but after that the light stay on i did replaced the thermostat but that made the car over heating now I also notice that my heater must of the time blow cool air any clue what us could be.
(2) Answers
(1) Comments
P0125 ECT Sensor Excessive Time To Enter Closed Loop
Possible Causes
•Check for low coolant level or an incorrect coolant mixture
•Check the operation of the thermostat (it may be stuck open)
•ECT sensor connector is damaged (check pins for damage, and for moisture)
•ECT sensor signal circuit has a high resistance condition
•ECT sensor is contaminated, damaged or out-of-calibration
•PCM has failed