Reading of "---" says to me that there is a bad connection somewhere. My manual says that the sensor is somewhere on the bottom side (maybe just behind) the front bumper. Guess you can check for a loose wire.

Display problems in the summer? How so? Remember that the car must be in motion for an accurate reading. If the car has been sitting in the sun, the reading can be influenced by warm air rising from the pavement. The sensor is faily low to the ground so it's likely the reading can be 10deg warmer than the actual temp when you first get in the car. Drive a short distance and you will get an accurate reading.

mormit
1985 245DL 233,000mi
2002 S40 7,500mi