A federal report by the Nuclear Regulatory Commission says the Japanese company that manufactured the troubled steam generators failed to meet some equipment testing requirements.
Inspectors claim Mitsubishi Heavy Industries failed to verify that more than 1,000 tubes being used in generator models matched specifications for tubing in the California generators.
The future of the San Onofre plant has been in the dark since January after a tube break released a trace of radiation.
Investigators later found heavy wear to hundreds of generator tubes. A three-month federal probe blamed a botched computer analysis for generator design flaws that ultimately resulted in excessive wear to scores of alloy tubes.
In light of their newest findings, inspectors are now asking Mitsubishi to explain how the problems occurred and to correct them.
Southern California Edison officials are in the midst of discussing plans with the NRC to reopen one of the damaged twin reactors.
Officials said a decision on that plan could come as soon as March.
The Associated Press contributed to this report.