GTX 345R Overheating?

[Sam Judd]

Hi all. I recently had a GTN 750xi and a GTX 345r installed. Yesterday the GTX345r worked for one flight, then showed the following error on the subsequent three flights. message: https://photos.app.goo.gl/v1XUCRnXAwqvi66w9. The 345r would display ADS-B in data (traffic/weather) on the 750, but would not move out of standby. I confirmed with other pilots that I did not show up on their ads-b traffic displays. Yesterday was unusually hot (43+ Celsius on the ground), the plane was left in the sun for a while and the GTX345r is unusually mounted just below the glare shield rather than in the avionics bay. 

Today I flew for about two hours and the transponder worked flawlessly.

Garmin has pointed me to the GTX 3X5 pilot guide, which has a bunch of detailed failure messages for the transponder. None of these are visible on the 345r and I think they're also not shown on the 750.  If I search for 'Transponder 1 needs service' here: https://static.garmincdn.com/pumac/190-01007-A1_04Web.pdf, it looks like a generic error message, which implies that the GTN 750 will not show specific GTX 345 failure messages.

My leading theory right now is that the 345r overheated, which would explain the transient failure and the fact that the transponder would receive, but not transmit.  It seems like the error message does not rule out the transponder going over temperature.

I'm working with Garmin and my local shop, but I have only two days before I'm supposed to leave for Oshkosh. Has any else experienced a similar failure?