That left Jang at 10-under 134, two ahead of New Zealand teenager Ko who carded a 67 that was highlighted by eight birdies but damaged by a double-bogey at the par-four 15th and a bogey at the par-three 16th.
Twice major winner Cristie Kerr and fellow American Jennifer Song were a further stroke back at seven under after posting matching 69s.
Though Ko stumbled over the closing stretch, she maintained her grip in the race with Korean Park In-bee for 2015 LPGA Tour honours as she seeks her sixth victory of the season.
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Ko is in pole position to clinch the LPGA Player of the Year award, the money list title and retain her number one ranking while Park has a slight edge in the chase for the coveted Vare Trophy for best scoring average this season.
Park, a winner of two major championships this year, returned a 69 at Tiburon to end the second round six strokes off the pace.
American Austin Ernst, who held a one-shot lead overnight after opening with a 66, triple-bogeyed the par-five first on the way to a roller-coaster 73 and a five-under total of 139.
(Reporting by Mark Lamport-Stokes in Los Angeles; Editing by Larry Fine)
