Outside temp is -20..Engine sounds rough. Advised to use HEET additive.. Also

That temp is hard on any car. Thickening oil, fuel that won't stay atomized, and engine parts that can't seal properly. Assuming there are no other issues with the car, a block heater wouldn't be a bad idea.

Unless it's really that cold outside you've got an issue with the ambient temperature sensor, or the outside temp sensor. It's located in the front lower grill so it's not uncommon to knock it off by hitting a curb or snowbank. I'm not familiar with HEET or a good block warmer however. Good luck, Bret