My engine fan is not kicking in, but my temp is between 80 -100. Is my thermostat bad because my check engine light just turned on

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My check engine light came on and it showed me a PO128 code which it has to do something with the thermostat temperature not right,,,but my cooling temperature is normal but, i notice the my engine fan is not kicking in what can the problem be???

My car has 106000 miles.
My car has an automatic transmission.

This can be caused by a number of things such a faulty thermostat, or a failing coolant fan switch. As you may know the coolant fan switch helps to maintain the proper coolant temperature by turning on and off at specific temperature thresholds. When this switch is not working properly, this can cause the fans to come on intermittently, all the time or sometimes not at all. When this happens you will notice a temperature spike and drop occasionally as the fan comes on and off. When your thermostat is not working properly or is stuck closed, this will not allow the coolant to properly circulate through the engine, which may cause the engine temperature to fluctuate erratically or in some cases just remain hot. I would suggest having an expert from Your Mechanic come to your home to diagnose your cooling system.

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