Temperature gauge occasional spikes and heat only works a third of the time

Hello -

Please make sure you let me know the size of your engine in liter and the 8th digit of your VIN. Also if it is just an Impala or a LS.

What was the reason you changed the thermostat..... Because of no heat or the temp gauge?

Check the heater hoses going into the firewall... Are both hot or one hot and one cold.

Please go to Auto Zone (AZ) or O'Reilly's (OR) and for FREE they can pull the codes to the car. Most important: Once they check your codes, if they find something and you don't get it fixed and need to get back with us, please make sure you tell us exactly what the code was, number and all. Example, if the code was E0568 O2 Sensor bad. Then make sure you give us all of that. While there for FREE also they can bring their tester out and check your battery and alternator.

What type of coolant do you have in it....... Prestone..... Dex-cool?

Hello my name is Dave.

My first thought is that you need to purge your cooling system to get the air out of it. Air in the system can cause both issues you are describing, intermittent heat and spiking temperature. Here is a link to our tutorial on draining and filling your cooling system, which covers the purging I strongly suspect is your issue:

https://www.2carpros.com/articles/coolant-flush-and-refill-all-cars

If you can make sure your system is purged so that we can eliminate or verify this as the cause of your issues. Please keep us up to date on your progress and thank you for using 2CarPros!