This error implies that a fan in your system is not working or is intermittent or is missing
open the case, and confirm that the fan that sits over the rear memory slots is spinning
this rear memory cooler shroud/fan as i recall is part# 663345-001 google for it to see pictures of it
you also have a rear case fan and a optional Front Cooling Fan Assembly part# 647113-001 that is required if using all ram slots and/or a high power cpu or all pci-e slots filled
make sure all fans are connected to the proper motherboard fan headers
confirm by looking at the z420 maintenance and service manual
https://support.hp.com/us-en/product/hp-z420-workstation/5225033/manuals