Problem with Stec 55x autopilot

Good morning members I have G1 SR22 with a stec 55x autopilot that is not holding altitude what could be the possible cause. The autopilot climbs and decent ok but when ever you engage the altitude hold it does not hold altitude. In altitude mode it climbs and decent by 3 to 4 hundred rpm

Join COPA and you’ll be able to search a treasure trove of posts on this topic, which will more than pay for the membership cost in saved labor.

It could be lots of things, all of which are somewhat common.

I had the same problem - the pitot static system was deteriorated.