Q: Check Engine code P0128

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The Check Engine light is on with the code P0128 for the coolant thermostat (coolant temperature below thermostat regulating temperature).

The cooling system temperature is regulated by thermostat, and the cooling fans order to keep it at optimal temperature for the motor to operate. It does this by keeping the thermostat closed and the fans off until the coolant reaches around 195 to 210 degrees then opens the thermostat to cycle the coolant through the radiator while the cooling fans blow through the radiator cooling the coolant. If the thermostat is stuck open then coolant will never stay inside the motor long enough to heat up to the specified temperature. This would set a P0128. To verify that the thermostat has failed a proper inspection of the Check Engine Light should be called out by a qualified technician, like the one from YourMechanic.

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