Engineers found that a vibration sensor might incorrectly stop emergency diesel generators following an earthquake.
The company reported the issue to federal regulators as "an unanalyzed condition that significantly degraded plant safety."
A vibration sensor detects loss of power following an earthquake, and switches power to generators in case of a loss of power. Power is vital to keep reactors cool to prevent a meltdown.
SCE said it has disabled the sensor. The equipment is being investigated.
SCE said in a statement Tuesday: "SCE personnel identified a vibration sensor at San Onofre Nuclear Generating Station that might incorrectly initiate a shutdown of the emergency diesel generators when a loss of off-site power happens at the same time as an earthquake. The sensor mechanism is designed to shut the diesel generators down during excessive vibration, which would indicate mechanical engine damage in the diesel generators."
The sensor has been in place at San Onofre since 1981. Edison said other backup systems were in place the entire time.
A steady supply of electricity is a critical issue at nuclear plants, which need power to control heat in the reactors. A tsunami destroyed backup generators at the Fukushima Dai-ichi nuclear power plant in Japan in 2011- setting off the world's worst nuclear disaster since Chernobyl.
According to records filed with the Nuclear Regulatory Commission, Edison engineers are looking into whether "high vibration ... could interrupt the onsite electrical generation" during a temblor.
If the generators fail, the plant can use battery power for up to four hours to operate the steam generators to cool the twin reactors.
The plant between Los Angeles and San Diego has been idle since January while investigators try to determine why tubing that carries radioactive water in relatively new steam generators eroded at an unusual rate, in some cases rapidly.
Edison initially targeted a June restart for at least one of the twin reactors, but that appears increasingly unlikely as investigators continue to review the widespread problem.
The Associated Press contributed to this report.
