Look to see if there is a name stamped on the spacer bar between the glass. The doors and windows are probably from the same manufacturer so you can check the windows as well.If that doesn't work, I would check with the builder or the previous owner to see if they know.
As far as replacing it, you can have just the glass replaced not the entire unit. Door glass sizes are standard in the industry and it is actually a pretty easy fix. Good luck.

Thanks eastbay. No luck on the spacer bar between the glass. just a black rubber basket. I also looked at the windows and discovered that they are made by Windsor. However, each of those windows has Windsor stamped on the glass with no reference to Guardian. My patio door is the exception.... no reference to Windsor whatsoever. Already checked with the builder and they "can't remember". Lots of help there. So I have to solve this one myself.

Find a glazier in your local area he will come measure the glass and install the replacement IG glass unit on site.

I HAVE A GUARDIAN PATIO DOOR 6' STATIONARY PANE THAT IS INCREASINGLY FOGGING UP. WHEN THE TEMPERATURE CHANGES. IT IS INCREASINGLY GETTING WORSE. MY HOME IS 4 1/2 YRS. OLD AND I LIVE IN A VILLA. WHAT IS THE WARRENTY ON THIS PRODUCT? I HAVE THE SERIAL NUMBERS ALSO

GUARDIAN DOES NOT MAKE THE DOORS,(stop shouting using all capitol letters)and Guardian will not warranty the glass you need to find out who made the door. You went to the trouble to search out a similar thread but then not read the answers as the answer was hear all along you just need to read!

"Find a glazier in your local area he will come measure the glass and install the replacement IG glass unit on site."