For some people that had this problem on the internet, they found that the plastic riser tray for the second CPU has sagged over time due to heat and stress. This causes the riser board to not make good connection with the motherboard and hence the PB7 (blinking 234 lights) error. This is why it suddenly happens without messing with the machine.
This can be tested by removing the second CPU riser board and seeing if the machine boots normally with only one CPU. If it does, then it could be the riser board connection causing the issue.
Their only solution has been to find some way to prop up the plastic riser tray to get it level again so the connection of the riser to motherboard is good again. They tested their solution by using some quarters to shim the riser tray to level it, and it would then boot with the riser and second CPU installed. Unfortunately, the quarters couldn't be left in since they interfere with closing the case, (and could dislodge).
The link below is to a post on how one person fixed their machine after doing the successful quarter test.
(Described in the second post on the page)
(Yes, it was posted on the Nissan Diesel Forums, but it has good pictures.)
Dell T7500 Precision - NissanDiesel Forums
Originally Posted by Ron Shurtz
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