After my original BGE dome thermometer got all out of whack, I just recently had it replaced by a dealer with a Tel-Tru off a new display egg.
I cooked a brisket today, and all through the cook, I had the egg held at about 250F, and the Tel-Tru and my ET732 were within a few degrees of each other. When the brisket came off the egg, I removed the plate setter, opened up the top and bottom vents (to put it in self-cleaning mode) and went back inside to finish making the french fries.
I kind of forgot about the egg, and half an hour later, when I remembered, I went out to shut it down and was puzzled to see the Tel-Tru reading 300F. I carefully opened the lid and knew it was considerably hotter than that. It looked perfect for doing a T-Rex sear. Still wondering what was up with the Tel-Tru, I foolishly reinstalled my ET732 grate probe, and it displayed 492F for a second, then HHH, at which point I snatched it back out of there, singing some forearm hairs in the process, even with my pit mitts on. While this was going on, and the lid was up, the Tel-Tru display came down to well below it's minimum normal readout, and I figured at best, I overheated it and it's going to need a recalibration. I shut the vents, and then checked on it 20-30 minutes later, and it was reading 400F.
Is it possible it's still working just fine, and when it was showing 300F, it was really well beyond 700F and had wrapped around to 300F again (maybe around 1300F if the dial scale is constant the second time around)? It's still cooling, and is down to 280F now.