As there is only one very busy avionics guy at my airport, it helps a lot if I can debug things on my own... hoping someone can point me in the right direction. On my last flight ATC kept telling me they were seeing me on radar at around 6000 but I was at 2000-2500 at the highest. I cycled the transponder mode but to no avail. I was "that guy". Oddly, this is the first time this has happened right after the avionics guy took a look at an unrelated issue (my air data computer not reporting the temperature correctly). I don't think he actually did any work or moved any cables but he may have. He says he can't be responsible for this altimeter/transponder issue but for reasons I won't go into, I don't entirely trust him. Maybe he knocked some cables or reset something? Where would I look first to get this sorted out? He's proposing installing a new pressure sensor and tying it to the transponder. Not a very complicated or expensive fix but I wonder if there's something else to check before jumping right into this and spending time and $. Avionics man is super busy and isn't going to do much debugging. I have a GTX-33 remote transponder and a GNS-480 GPS that controls it. The altimeter is a KEA 130A encoding altimeter, I believe. Aircraft is a 1986 Mooney 252/Encore conversion.