Thanks for the explanation Steve. I was chuckling- give the coders smart "black tape" to cover the annoyance! I get it. The visual load on the pilot is sometimes high. Why not suppress the light when it is unimportant. Light on or light off, I don't have a strong preference for either behavior, but I do have a preference for accurate manuals so I know what to expect.
On the other, the pressure altitude, IMO, if ATC asks me to shut of mode C, I'd rather see the light come on. Those of us opting for UAT will likely spend a good deal of time, 1200 anonymous. I'd prefer to have the light if my out has a problem. Consider that this UAT unit is targeted toward those who largely want to keep their existing transponder and blind encoder. We are talking old equipment in many cases. Blind encoders do fail and I've seen poor connections as well. Given the structure of Gillham code, when there is an error, it can translate to a wrong altitude that is off a great deal more likely than off a few hundred feet. As long as the team is building smart fault lights.... any thought to a compare statement between pressure altitude and GPS altitude? It would need to have lots of fudge, extreme example, IIRC about 4,500 feet pressure altitude at sea level during record low pressures (hurricane). GPS and pressure altitude seldom agree, but with all the smarts at Garmin.....
In the big picture, probably not a big deal to ignore pressure altitude faults. Most users will probably have some kind of portable IN (that is how I saw the behavior that was contrary to the manual). So if the fault light doesn't come on, perhaps it will be noticed in the "own ship" values. But also big picture, why add a fault light if you don't allow the code to trigger it per the manual- anytime the GDL is not broadcasting FAA required information?
Bottom line for me, whatever Garmin does with the fault code, put it in the manual so we know what to expect.
1990 RV-3 (now apart, upgrades in the works)
1959 C172 O-360