On Mar 11, 7:49 am, "Dico" wrote:
Hello,

I've noticed during a flight the other day to Columbus that our Mode C
was reading high... by about 250 feet. The altimeter seems to be
indicating fine as on the ground, with the correct altimeter setting,
it shows the correct altitude. As well, flying the ILS, i make a habit
to check the altitude when crossing the FAF and it is correct too.

Any thoughts on what the issue is?

1) the attitude from your transponder is pressure altitude, ATC
corrects for pressure.
2) I learned long ago not to try to debug issues with transponders,
they seem to be well out of the range of non-radio,electronics people.
However, generally adjustments are very inexpensive. In fact, most
places that quote fixed price IFR certs include basic adjustments to
the transponder. Every time I bring my plane in for IFR cert they have
to do some tuning of the unit.

-Robert