Is the certification process a big barrier to fixing the problem? I mean if they want to fix it, they presumably have to go through certification again. Is it really that big of a problem to do?
Normally a big firm like this has the authority to do mods without reference to the FAA.
I think there are perhaps several reasons they don't fix stuff:
1. All people who knew about it have left the company (true for most Honeywell products)
2. The product was bought in, or developed by a contractor who is no longer around to fix it (true for e.g. the KSN770)
3. The lawyers have banned any acknowledgement of the issue, so also banned fixing it - unless the fix can be done under the guise of some harmless sounding SB. This is a
huge problem in GA as a whole. I know that e.g. Honeywell have gone to great lengths to tell individual customers that they are the only people with a problem, or that it happens only on the one aircraft model.
4. The company is no longer interested in anything beyond running some old cash cows.
STEC autopilots were never much good for performance because they use the TC as the roll reference, and use indirect means as the pitch reference.
Also the number of STCs which STEC got is so huge that it is totally obvious that they could not have possibly tested the system on every aircraft
and done so at all the limits of the loading envelope
and done each of these at both ends of the airspeed range. That's why we get so many reports of STECs oscillating. The KFC225 in comparison almost never had any problems and delivers really accurate control; its weakness is the servos.