If the HPU oil overheats: Some HPU assemblies have fans installed on the back. Make sure the fans operate when the spindle is in operation.
Make sure the spindle motor fan operates.
If the machine has a heat exchanger, make sure the heat exchanger is clean and its fan operate correctly. Not all heat exchangers have a separate fan. Some machines have remotely-mounted heat exchangers with integral fans.
The spindle fan on machines with the Classic Haas Control with software version 11.27A or higher and all Next Generation Control machines must stay on when the HPU is on.