Seldom will a faulty sensor or display result in that small of an "error", with that consistency.
An easier and non-intrusive test is to compare the display temperature against a digital thermometer with a probe ($15). Place the probe as close to the sensor as possible. Be sure to do this comparison when the engine is cool and the front bumper is in a stable temperature environment, out of the sun. Chances are that you'll find the two readings within a degree or so.
Remember that under normal conditions the outside temperature is being taken in relatively close proximity to the radiator, the warmer pavement, exhaust from other vehicles, etc. You need to (mentally) adjust for these influences when comparing your display reading with the "real" outside temperature.