"Earlier this morning, an equipment malfunction caused the safety system within one of the units at the Commerce City Refinery to activate," a company spokesperson said.
"The system worked as designed, shutting down the equipment to prevent any further disruption."
Suncor reported a similar incident on March 22 at the refinery, which was restarted in early April after completion of maintenance work.
Suncor also reported a fire in its gasoline-making unit in early May.
The facility comprises three plants from two refineries and is the largest in the Rocky Mountain Region, the smallest of the five U.S. petroleum districts.
There was also no impact to the community following the incident.
(Reporting by Seher Dareen, Ashitha Shivaprasad and Arundhati Sarkar in Bengaluru; Editing by Louise Heavens and Mike Harrison)