Just an FYI for everyone with a 600U.
I have had a Panasonic TH-50PX600U for several years now and a I am supremely happy with it.
On the day before Valentine's day of this year, it lost its ability to switch to the TVGOS screen. This happened overnight while the TV was turned off. Anything I would try to get the TVGOS function to display would fail. It would act as if I never pushed the GUIDE button at all and the TVGOS feature never existed.
This led me to do a master reset on the TV but it didn't help any. TVGOS was still gone.
I did check in the TV's menu system in the "About" category to verify the TVGOS firmware version but it showed BLANK. The set's other firmware versions showed correctly however. Just the TVGOS firmware was gone! The TV acted completely normal in all other respects.
So, I gave Panasonic Tech Support a call and spent hours with 3 different technicians to no avail. I reloaded all of the TV's most recent firmware available from Panasonic numerous times but it never made any difference. They couldn't even guess as to what could be the cause of my bizarre problem. In the end they suggested I take it in for service. I really didn't want to do that.
To make a long story short, seeing as how I am electronically inclined, I decided to take a look at the Panny's service manual to see if any mention was made of TVGOS, but there was no reference to it. I did find that the most likely board to contain the TVGOS firmware was the DT Board
(Handles tuner functions). I found a used one on eBay for an insanely cheap price, so I took a chance and replaced that board.REPLACING THE DT BOARD SOLVED MY TVGOS PROBLEM.
As soon as I powered up the TV after the board replacement, I went and checked the TVGOS firmware version, and it was displaying properly. All TVGOS functions worked perfectly once again.
Since TVGOS firmware occasionally gets updated automatically while the TV is turned off (in addition to the normal Guide data), I suspect that it accidentally erased the existing firmware and there was no way to reload it.
Sorry for the long-winded, dry read. Just thought I would share my experience with others.