When you say it is really cold, how cold is that? If you are in the northern states when it is significantly below 0 then you may be in need of a block heater so that you can build the heat in the engine that you need.
Assuming that the heater core was replaced recently then we can assume it is not the issue. However, I would check the temperature of both heater core lines at the fire wall and just make sure they are the same temperature. If not, then you still have a restriction in the heater core or lines.
https://www.2carpros.com/articles/replace-heater-core
If this is okay, then we need to check the cable and make sure then when it is cold that it is moving the door all the way to hot. If not, then you may need to adjust the cable.
Please run through this info and let me know what other questions you have. Thanks
Image (Click to make bigger)
SPONSORED LINKS
Sunday, November 7th, 2021 AT 7:11 PM