The temperature reading displaying ——— is often caused by a bad thermostat. The computer on these cars is programmed to have the temperature rate of change happen in a certain amount of time. If the temperature doesn’t rise fast enough, as when a thermostat is stuck open, it’ll show ———. I’m not quite sure if the same thing happens when it’s stuck closed or not as the temperature may rise but not flow coolant into the area where the sensor is located. Which is right beside the thermostat. Stuck closed would also cause an overheating issue. Occasionally it is a sensor causing the issue. But I have fixed more or this particular problem with a new thermostat.
I would suggest by first making sure the coolant is full in the reservoir. If it’s not empty or extremely low, it may be time for a thermostat. I highly recommend you used a quality thermostat and don’t cheap out by purchasing the cheapest one available. Many times the cheap ones either don’t open at the correct rate or the opening temperature is slightly off and can also cause the ——— to display again or other issues.
I personally used a Stant OE type thermostat. It wasn’t the most expensive, but wasn’t a cheap piece of crap. Most thermostats available now days are either made in China or Israel from what I’ve purchased for various customers vehicles. Stay away from the Motorad Fail Safe thermostat, they seem to always get stuck open. Their premium ones are ok.
I also flushed the cooling system out completely and filled the system with Prestone yellow/green coolant to replace the orange Dexcool. I have had many instances where I’ve worked on GM vehicles and found the Dexcool with sludge. I assume it’s not just the Dexcool alone causing problems, but I will not used the stuff in my personal vehicles. I've used Prestone coolant for years and had no adverse problems in the cooling systems I’ve used it in. Just my preference. And the stuff isn’t really very expensive. Also can be found at most auto parts or grocery stores, so easy to access.
try a piece of hose split it then a couple of zip tiesWhile you’re under the hood, make sure the wire loom hasn’t worn away on the wiring harness that runs next to the oil filter housing. The sharp edge on the engine will cut right through the wires and can cause many issues. They sell the wire loom at most auto parts stores. I still need to find the larger size for the larger bundles of wire.