Transponder check

On Thursday, June 6, 2019 at 7:53:51 PM UTC-4, wrote:
it occurred to me, that the previous day I had obtained a free FAA report on my adsb/transponder in flight, which was satisfactory.

In another thread, a 'known to be knowledgeable person' said that the obligatory 'transponder check' tests things that are not tested by the Public ADS-B Performance Report.

On a related note, does ADS-B out report the GPS altitude, the pressure altitude reported by the transponder or both? I assume that a Mode-S transponder is still talking to TCAS systems and not using GPS altitude. What happens when the pressure altitude from the transponder disagrees with the GPS altitude? Would this allow the transponder to detect a blocked static port or bum pressure sensor?

My 'transponder check' was done inside a metal hangar building, so there was no GPS altitude available to compare to the pressure sensor altitude.