GTX 335 altitude routinely off by 100'

Over the past year I've noticed the pressure altitude reported by the transponder is routinely off by 100' when compared to my altimeter when set to 29.92. In every case it reports me being 100' lower (never higher). On some rare occasions it's been 200 feet (since there is only 100' resolution on the GTX335 I can't verify the exact delta).

I just got my 91.411 checks done in July when I got the G5 AI installed (and those checks included the transponder), and it says it checks out within tolerances.

Questions:
1) Any thoughts/ideas on which components could be causing that? The transponder unit itself, altitude encoder, or a problem in the static system? Is there a culprit that is more likely than others given that my main altimeter/G5 altimeter seem to work fine?
2) If I'm reading the 91.411(e) docs right, the maximum deviation allowed between what the altimeter shows and what the transponder shows is 125'. Is that the correct interpretation? After it exceeds 125' I would have to bring it in and get it re-tested/calibrated?

 

Being intermittant it sounds more like standard ops while flying. Two different instruments "reading" the same input in their own method. If that makes sense.

No. The 125' variance is for a controlled test environment not in flight. No further work needed. Now if there was a continuous difference from ground to flight levels that would at least require a look.

 

@GeorgeC : I think it was installed in 2017 (prior to when I bought the plane)

@Bell206 - thanks that makes sense. But to clarify, there is a continuous difference from ground all the way up to the highest I fly (usually about ~10k'). So not the flight levels, but certainly consistent. Is that still concerning and worthy of a look?

 

No. When you take in consideration all the combined test tolerances between the various equipment you will have variances in flight. I probably should have said when the tower calls you out on altitude or there is a marked difference then talk to someone about it. If you want at your next 411/413 check bring it up to them. Sometimes with these new digital instruments you can get too much information at times. But it never hurts to ask.