The following article contains information about Dell Precision tower system not powering up with the side cover removed.
The Dell Precision 5820, 7820, 7920 Tower and XL Tower systems need the side cover on the system in order to operate. If the side cover is removed while the system is running, the system will immediately (within 4 seconds) power down. If the side cover is off when powering on, a Power Button LED error code of 4-7 appears.
This behavior is normal and is working as designed.
If you have further questions about this article, contact Dell Technical Support.