How to use HP Proliant DL380 with only 5 fans (instead of 8)

Some people never learn the meaning of "mind the gap".

When you reset the BIOS or change the motherboard on a HP Proliant DL380 server, the automatic motherboard protection feature doesn't let you boot the system as it feels few fans are missing.

There is only one possible way of solving this problem without spending 150$ (x3) on each so called missing fan.

The server supports redundant hot-plug fans to provide proper airflow to the system if a primary fan fails. The server airflow is divided into three zones:
processor, I/O, and power supply. Shared fans 4 and 7 each provide airflow for two zones. If a shared fan fails, the failure is experienced in both zones.

In the standard configuration, five fans cool the server: fans 2, 4, 5, 6, and 7 in the processor zone; fan 4 in the I/O zone; and fan 7 in the power supply zone.

For the redundant configuration, fans 1, 3, and 8 (one fan per zone) are added to cool the server. This allows the server to continue operation in non-redundant mode if only one fan fails per zone.

The server shuts down in the following scenarios:

• At POST:

• The BIOS suspends the server for 5 minutes if it detects a cautionary temperature level. If the cautionary temperature level is still detected after 5 minutes, the BIOS performs an orderly shutdown and then restarts. This process repeats until the cautionary temperature level is no longer detected.
• The BIOS performs an orderly shutdown if the following minimum requirements are not met: one working fan in the I/O zone, five workingfans in the processor zone, and one working fan in the powersupply zone.
• The server performs an immediate shutdown if it detects a critical temperature level.
  

IMPORTANT: An immediate shutdown is a hardware-controlled function and it overrides any firmware or software actions.

Here is the solution:
As seen from the table below, some fans are primary and some are redundant. If you only socket your fans into the primary slots, leaving the redundant ones empty, the system will boot up normally.

At this point I'd strongly advise you to enter the BIOS settings (F10) at post and disable the automatic overheat shutdown function as this function shutsdown the system without even checking the heat difference. No worries, as it says above, even if this function is turned off, its still active and it will switch the system off if the heat level goes critical.