The sensor for the outside temperature in located in the lower bumper just inside of the left fog lamp. It's black plastic about 3/4" in dia. with a pointed end. Start there to check for the problem. Also there are bulbs which illuminate the instrument cluster. If the lower left one is burned out, the temp is very difficult to read especially if the illumination is turned down. Repeatedly press the '+' button to see if there's a reading. To remove the cluster (friction fit) you need a set of cluster hooks.

A more serious problem is 'missing pixels'. A few people have worked on the ribbon cable which connects to the display with varying levels of success. If necessary there are a couple of companies which do repairs.

The left fog light is an issue for W210's as well as the left stop light. Actually, getting false positives on burned out bulbs is pretty common. If the light is out but the bulb and fuse are ok, try cleaning the contacts. Next examine the socket and housing for signs of overheating. You'll need to remove the front anti-noise panel. If the socket is shot, MB makes a repair kit which includes the socket and a pigtail to splice into the harness. Although, some newer models have a quick connect.

Good luck.