It's similar to a problem found at the Unit 3 generator.
San Onofre has been shutdown now for two months.
The Nuclear Regulatory Commission says the plant will remain closed until investigators can determine the cause of the wear in the equipment, and then fix it.
The new findings come just days after NRC Chairman Gregory Jaczko signaled that Unit 2 might be able to go back into service more quickly than its twin.
The Associated Press contributed to this report.