The parts were spotted by CNBC outside a shop called JL Precision, not far from Tesla’s Fremont factory. They included door frames and a variety of other components shipped from suppliers in China and Ohio. Unnamed sources within Tesla told CNBC the shop was used to rework designs or correct flaws in components.

But Tesla spokespersons have strongly denied those claims, insisting that JL Precision does not rework flawed parts, and only applies coatings that are a normal part of production.

Outsourcing the fixing of flawed parts is common practice in the auto industry, according to a former GM plant manager who spoke to CNBC. But Tesla appears to be dealing with a higher than average ratio of problems, with one engineer there estimating that as many as 40 percent of parts manufactured by Tesla or its suppliers required fixes.

Multiple current and former Tesla employees told CNBC that Tesla spent less time vetting suppliers than is standard in the auto industry, and that some of those responsible for the screening were not experienced with ISO standards and other quality assurance methods normally used in that process. Tesla has disputed those statements as well, insisting they have a rigorous supplier screening process. Tesla has also said that parts fixes aren’t adding to delays in Model 3 production.

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