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Q: P0128 - Coolant Thermostat had a fault 2007 Nissan Sentra

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I have 2007 Nissan Sentra 2.0 which has engine light on Error Code - "P0128 indicates that the coolant thermostat had a fault (Coolant Temp below Thermostat regulating temperature) for a predetermined period of time." Autozone just looked in the system and suggested for a Sensor replacement. I also took this to another service place. They said to take it to Dealer and it could be because the transponder key is not functioning correctly which can be programmed only by dealer. Both of them completely out of sync.

Symptoms - Heater takes at least 10 minutes to start giving hot air. Fan working perfectly fine at different speed levels. Any suggestions - Do I need to replace thermostat or sensor or anything else I should diagnose? Any suggestion? Thank You!

My car has 117000 miles.
My car has an automatic transmission.

Hello - your thermostat is not functioning properly, and should be replaced. The problem code is indicating that the engine temperature is not coming up as quickly as it should - which the thermostat controls. You have noticed this in how long it takes the heater to be functional. I would recommend a thermostat replacement](https://www.yourmechanic.com/services/thermostat-replacement) by a mobile, professional mechanic, such as one from YourMechanic, who will come to your location, and correct this problem.

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