Leon,
  FYI, I mounted my temp sensor on the bottom of my left wing right next to
one of the idler arm access panels.  I calibrated it according to the RMI
manual and it's always been within 1-2 degrees of the temp reported on ATIS
or AWOS.  If you haven't calibrated yours, I'll bet that's the problem.
Just get an accurate thermometer to place next to the probe to use as a
reference and follow the instructions in the manual.  You can display the
temp in C or F.  I calibrated mine in farenheit because that's what my
thermometer read, then set the instrument to display centigrade, as that's
what all the field reports are in.
  Skip Slater