Ok so I talked with Pac Sales. They said the Thermador rep says the backguard is not necessary. The rep thinks there is a defect with my machine. They're scheduling for the service dept to come take a look. Supposedly 6 years ago when the machine first came out there was a defect that could cause this issue.
However, when I called Thermador service dept directly, they say that they ALWAYS recommend a back guard when installing against the wall because no one can be sure if the material on the wall is combustible or not. So the Pac Sales/Thermador rep and the customer service guy are giving conflicting messages. I trust corporate over individual sales to be honest.
Good thing is that Pac Sales says that if it turns out the issue is caused by the oven, they will repair the back splash or do whatever to fix it. We'll see how it goes after they come over.
I have to agree with Fred, I thought this was just a practical issue, but turns out it's a safety issue. The manual specifically say that if there is no back guard, I would need 12" space between oven and combustible material; if there is a backguard, I need a small gap (an inch or something). The sales guy thought stone/quartz/tile count as non-combustible when it doesn't. I believe they are wrong.
Q
Thermador issue
Q