Hi Bajil, Have a diagnostic scan carried out and have the coolant temperature sensor and relay inspected for correct operation. It sounds as if a cable has become disconnected or that the operating circuit has become defective. The cooling circuit operation is correct when the fan operates when the engine temperature raises a fraction above normal. The fan will run for a number of seconds and then switch off, thereby maintaining the 'normal' operating temperature. Regards John
SOURCE: I HAVE A 2007 CHEVY COBALT THE CHECK ENGINE LIGHT
I had this problem with a 2007 Chevy Cobalt. I changed the coolant sensor and even though the check engine light went out I still had a reading of "---" on the coolant gauge. The problem is most likely one of three things: a low coolant level, a bad coolant sensor, or the thermostat needs to be replaced. The only other possible thing it could be is a bad BCM (Body Control Module) which in that case you would most likely be having other weird problems.
The Thermostat and the sensor are both very easy to get to and replace. Take off the engine cover by removing the oil cap and they are both to the rear of the engine on the driver's side. They are right next to each other.
Hope this helps!
SOURCE: ENGINE TEMP GAUGE READ 3/4.
Is there a vast difference in the temperature between the upper and lower radiator hoses? If there is then you should check out the water pump (since you already changed the thermostat).
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