During installation

If an ESXi 5.1 host does not successfully install, serial line logging can be configured when booting the installer using advanced kernel boot-time parameters. This takes effect for the current boot, and does not persist across future reboots.
To enable serial-line logging during installation:

1. At the Loading ESXi installer screen, press Shift+O.
2. At the prompt, append the advanced option to specify a serial port after runweasel:

   > runweasel debugLogToSerial=1 logPort=com1

3. Press Enter to boot.
4. Logging starts after the first loading progress bar finishes.

http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=2032350