I’m flying the Cessna 172. I’ve programmed the “Garmin 530” for the RNV 33 GPS approach to KISM with YONMA as the IAF. On the GPS/Nav switch I choose GPS. On the autopilot panel I engage nav with gpss and altitude hold. With this configuration, the plane follows the course precisely. However, as soon as I engage the approach button, the plane immediately deviates from course. This happens no matter where on the approach I am. If I don’t engage the approach button then the glideslope will not capture, altitude hold will not disengage and the plane will not descend The same thing happens on an ILS approach.