2008 cobalt,check engine light on temp showing slashes instead of temp coolant was a little low filled it light still on still showing slashes instead of temp

SOURCE: 1997 2.4L Chevy Cavalier LS; Low Coolant Indicator and Temp Gauge

take the cap off of the radiator (whil engine is cold) start and run the vehicle. keep runnig 30mins after the vehicle is hot keeping notice of the rad fluid level.
keeping the cap off will prevent the pressure build up that woiuld keep the air in the system, keep adding water or antifreeze as the level goes down. this may take an hour or so.
also with the cap off, you will be able to see if your water pump is operating correctly. you will visibly be able to see movement in the rad when the pum is engaged.

SOURCE: 2005 chevy equinox temp gauge showing hot fan

check the coolant temp sensor

SOURCE: 2006 HHR, check engine light came on, the engine

Your coolant temp sensor is not registering. It probably set a trouble code, that's why your remote vehicle start is not working. The temp sensor is located at the rear corner of the engine block on the back side of the engine. This will be the corner of the engine closest to the driver. The temp sensor may be faulty, unlugged or there could be problem in the circuit. Even if you find the sensor unplugged and you plug it in, the code will have to be cleared before the remote vehicle start works again.

SOURCE: 2002 Nissan Sentra 1.8L Check Engine Light Low Coolant

check out the coolant temp sensor.

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!