I agree with Hwnmurano, temp seems to show about 10 degrees higher than the real temp.( Of course this is more noticeable when the car is sitting in the driveway baking in full sun.) The heat reflecting from the asphalt and concrete must be sending bogus readings to the sensor. I do notice the winter cloudy day readings to be much more acurate. Guess you guys in Hawaii will never see those acurate 20 degree readings! Ha!
Mine reports about 10 degrees higher, if the MO has been sitting idle for a long time.. it takes about 5 mins to get to the correct temp. (it would make sense as it's sitting behind a BLACK cover which will get hotter in the sun, when it's sitting idle)
Mine's pretty accurate (compared to the temp reported by the radio for the entire metro area). I do notice that if the car sits for a while, the temp is a few degrees higher (not as much as 10F though). Makes sense since sitting in the sun will cause the car to bake. It comes back down after a mile or so of driving. If your temp reading is consistently ten over, it needs to be recalibrated or replaced.
I recall reading in the service manual that the ambient temp sensor has change limitations built in to it's operation (like 2 degrees per minute) so that it can't adjust dramatically in reaction to outside temp changes.
This is done supposedly so that it doesn't report the significant increase in temp at the sensor when your stopped in traffic.
I think it was only limited on the going up side but can't recall for certain.