LOS ANGELES (AP) – A major transmission line feeding electricity to millions of Southern California customers shut itself off because of a faulty sensor, triggering scattered blackouts in the middle of a heat wave, officials said Friday.

The Thursday afternoon outages, lasting about 30 minutes, affected about 500,000 Southern California Edison customers in several communities east and south of Los Angeles.

“There was an oil flow alarm that went off,” said Carol Tucker, a spokeswoman for the Los Angeles Department of Water and Power, which co-owns the transmission line.

“Upon checking into it, we determined that there was no problem with the actual converter-transformer,” she said.

The sensor was working normally Friday, Tucker said. State energy regulators said further outages were unlikely now that the problem had been identified. The cause of the sensor malfunction remained under investigation.

The blackout was a jarring reminder of the energy crises that struck California in 2000 and 2001, when energy bills skyrocketed and the state suffered several rolling blackouts during a time of limited power supplies.

Energy regulators dismissed the idea that the latest outage could suggest California’s energy reserves were being overtaxed by the heat wave.

“The demand on the system was higher than we anticipated, but not so much that it was causing a drag on the system or that it was causing any problems,” said Gregg Fishman, spokesman for the California Independent System Operator, which operates the electric grid.

Temperatures that hovered around 100 degrees in inland areas Thursday created increased power demands of about 1,500 megawatts, officials said. A megawatt is enough power to power about 750 homes.

“There was enough power to meet that demand until we lost the major resource on the transmission line,” Fishman said.

The sensor that went haywire monitors the flow of oil inside an electric power transformer north of Los Angeles. When the unit turned itself off, officials were forced to cut power in Fontana, La Puente, Cathedral City, Huntington Beach, Long Beach and Ontario.

The converter is one of two used in the Sylmar station to distribute up to 3,000 megawatts of power imported by California from the Pacific Northwest.

No break from the sweltering heat was expected over the weekend, with temperatures as high as 108 degrees forecast for inland areas.