The chiller uses its condenser as one of the walls of the enclosure. The air pulled through the chiller by the fan must go over this condenser to remove the heat from the refrigerant. If an access panel is open, the majority of the airflow will circumvent the condenser through the unrestricted panel door opening. Not enough heat will be removed from the refrigerant and the pressure will start to increase eventually reaching critical safety levels causing the unit to shut down. By replacing the access panel while the unit is in operation you can avoid this issue.