The fire started around 11:30 a.m. Saturday near I-15 and Deer Springs Road, the California Highway Patrol reported.
There was an early report that the fire was holding at 25 acres from Southern California Air Operations, but Cal Fire San Diego spokesman Thomas Shoots said the correct acreage burned was seven acres, not 25.
Fire investigators found debris from a faulty catalytic converter in the area, which is suspected to be the cause of the fire, Shoots said.
"Overnight, crews were able to increase containment to 80%. We will remain at scene today to continue mopping-up and containing the fire," Cal Fire San Diego tweeted at 8 a.m. Sunday.
At least four North County firefighting agencies and one from Riverside County joined Cal Fire to fight the fire, including the Escondido, San Marcos and North County fire departments, San Diego Fire-Rescue Department and Pechenga Fire Department from Riverside County.
So Cal Air Operations tweeted at 2:30 p.m. Saturday that air tankers and ground sources were released, transitioning to patrol status. One copter continued to assist ground crews with hot spots.
Two lanes of southbound I-15 south of Gopher Canyon Road were reopened, Caltrans San Diego reported.
There was no immediate threat to structures, according to the San Marcos Fire Department.
