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?

The altitude your mode C reports comes from a separate pressure
transducer, not your flight instruments, so the accuracy/precision of
your mode C altitude is not related to the accuracy/precision of your
flight instrument.

The mode C reported altitude is pressure altitude, so needs to be
corrected for barometric pressure for comparison with your flight
instrument. The altitude shown on ATC's display is corrected to the
local altimeter setting, but the raw data from your transponder is not.

Mode C altitude typically has a resolution of 100 feet, which gives rise
to possible sizable rounding errors. Newer pressure transducers used
with TSO146 GPSs have a resolution of 10 feet.