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?

Thanks,

Dico

You could have water in the line. This usually causes strange readings.
If it is past the Altimeter then it will not affect it.

Michelle P