Most likely, it's one part supplier, machine or machine operator that had a history of this defect happening. They know every aspect of the assembly of each vehicle (which employees were running things, which batch of materials, which exact machines were in use on that particular day, etc). Probably saw these problems, and kept seeing the same exact factor with all of them. Even if it's a few dozen, they have to then check every single one where these factors matched up.For context, this is only 0.5% of all vehicles made over six years. This is only 4,700 vehicles. Last month that factory made 30,000 vehicles. Honestly, the fact Ford was able to find a common denominator is pretty amazing.
Maybe the machine that torqued a bolt was getting out of a spec and no one knew it, who knows.
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