When I first entered the auto industry, I was perplexed by the question of why suppliers are tasked to perform their own testing. In other words, if I supply a window control module, then I am responsible for providing documentation that my module passes all tests. Until I produce this test report, I don't get my PPAP money. This is the equivalent to the Ivy League saying, "our profs don't have time to correct your exams, so go do it yourself and tell me your final grade"
No points for guessing that the 6th layer of Hell is dominated by test technicians and their managers.
There are two ways suppliers test their product:
Most major suppliers have their own test facilities. Therefore, the staff who determine whether a product "passes" or "fails" are paid by the same company that makes the product. So let's say it's a typical Friday afternoon and while you're running a radiated immunity test, you spot a slight twitch in the motor. Nothing too serious but the test plan clearly states "no momentary reaction." So you have 2 choices:
1) Not notice it and go home to a nice warm supper.
2) Take note of it and have the whole of the company's brass descend on your department, demand hourly updates and plan out how you're going to spend your next 3 weekends.
Smaller suppliers are compelled to outsource their testing. So let's say that it's the same typical Friday with the same motor twitch, only this time you work for an independent test lab. Your two choices are:
1) Fail the test. Send email to supplier telling them to come and collect the twitchy failed parts. You prance to your car knowing that you've created enough overtime income to take your two teenage daughters to see Justin Beiber live in concert.
2) Pass the test, knowing that if this problem ever happened in the field and resulted in a recall campaign, it's your head that everyone will be after.
This system, has another inherent flaw. Testing is very expensive. Passing tests is even more expensive. The right balance needs to be struck between running enough tests to capture the bulk of potential failures versus testing for every imaginable condition and failure mode on the face of the earth. It's easy to say "let's test for everything," except that when you do so, it is not only the cost of testing that increases. Now the cost of preventative measures for those one-in-a-million failures is tagged on each and every part. Extreme excellence is what car owners would like to own but not what they want to pay for.
So when a car manufacturer does not need to run component tests themselves, it's easy for them to start piling up test after test with extremely stringent requirements and often overlapping environmental conditions. In their world, they are simply striving for excellence. In their suppliers' world they are the customer to whom we quote 25% more in development costs.