During the winter the heat is lukewarm

Hello,

Yes, it could be the thermostat, or it could be a few other things as well, but I would recommend starting with the thermostat. Down below is a link to a general diagram on how to replace a thermostat:

https://www.2carpros.com/articles/replace-thermostat

In the diagrams down below I have included the instructions for the removal and installation of the thermostat on your vehicle, as well as the location, instructions for testing, and the Original Equipment Manufacturer[OEM] part number for reference as well. Please go through these guides and report back with how everything turns out.

Thanks,
Alex
2CarPros