Tesoro Corp. (TSO) experienced a snag Monday at an unidentified unit at its 100,000-barrel-a-day Wilmington, Calif., refinery, leading to unplanned emissions, according to a California regulatory filing.

The unit unexpectedly “depressured” and released sulfur dioxide from a flare stack.

The Wilmington plant is located in Los Angeles County.

Tesoro shares were recently up 6 cents, or 0.4%, at $14.99.