I think the main reasons mechanics don't like using your parts is because they won't be able to honor any type of warranty on it. If they use their own part and it breaks, it is on them to fix it. If they use your part and it breaks, not due to installation, then you're still going back to them and complaining that they didn't fix the car correctly. It's a weird situation for them as a customer's part broke, but the customer is always right, so they are the ones that "broke" the part.

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