[arcticdude]

If yours is a 431, you should be able to remove the cover off the main burner box by drilling the rivets out. The fiber insulation will be attached to the cover, so once removed the burner/coil assembly will be exposed. There's a hard thermal conductant that helps heat transfer between the coil and burner. If your sensor is there, it may be on the opposite end from the thermostats, as that end does not have anything impeding contact. But that would be a long way to go for a temp sensor. If it's measuring the fresh water temp, it might be located right on the coil, which is partially exposed (once the main cover is off).