Cause : Heat-sink over heat comes when the heatsink temperature reaches the overheat fault level. The "oH" fault indicate that the drive cooling fan is not working, ambient temperature is too high or the drive heatsink temperature sensor is malfunctioning. The drive will display "oH" and stop operation. 

1. Ambient temperature too high.
2. Fan is damaged.
3. Lower the ambient temperature.
4. Fan air duct is blocked- clear the air duct.