DFC;
In the US we do require that altimetry and Mode C be checked I beleive every two years. If we find that someone has a bad mode c while we are talking with them, we can tell them to turn off the mode C so that it doesn't give false RA's for TCAS. We also are supposed to turn in a report to flight standards so that they can check with the pilot to ensure that the aircraft gets the altimetry or the mode C fixed...
regards
Scott