The Nuclear Regulatory Commission report raised the possibility that Mitsubishi Heavy Industries was looking for fixes for the damaged generators on the wrong-sized test equipment.
Government records indicate the NRC issued a "notice of nonconformance" on Nov. 30 after inspectors visited a Mitsubishi plant in Kobe, Japan.
The U.S. regulators found that workers failed to verify that more than 1,000 tubes being used in a generator mock-up to explore potential fixes matched specifications for tubing in the sidelined California generators.
Inspectors also found that a gauge used to measure strain on tubing was not properly certified, NRC spokesman Victor Dricks said Monday.
The agency wants the company to explain how the problems occurred and when the issues can be corrected.
The inspection came as Mitsubishi was preparing to use results of the testing done on the mock-up generator in other research, the report said.
San Onofre operator Southern California Edison referred questions to Mitsubishi, which had no immediate comment.
The seaside plant between San Diego and Los Angeles has been offline since January, after a tube break released a trace of radiation.
Gradual wear is common in such tubing, but the rate of erosion at San Onofre startled officials because the equipment is relatively new. The generators were replaced in a $670 million overhaul and began operating in April 2010 in Unit 2 and February 2011 in Unit 3.
The plant is owned by SCE, San Diego Gas & Electric and the city of Riverside. The Unit 1 reactor operated from 1968 to 1992, when it was shut down and dismantled.