You know, itís such a shame. I am a professional and my job has to do with design. If a client comes to me asking me to solve a problem then I design something for him and after he orders it he finds out it doesnít work, guess what? I will bear the responsibility and costs. Itís not his job to know if the design works or notóitís my responsibility because I am the designer and he came to me! Same thing with mechanics: a customer doesnít know why his car shakesóthatís why he came to you! He doesnít know if itís the drive shaft, the wheels, the whatever, or even if the chassis needs pulling or not! You are the mechanic and itís your responsibility to diagnose the problem correctly. If you donít know then say, ďI donít know. Sorry.Ē There is no shame in not knowing. But if you claim to know and you are wrong then you should bear the costsóall of it (parts and labour). Donít rationalize it or try to cover it up, or pretend it didnít happen. Or try to explain how the change was beneficial. If it did not solve the problem then it was no beneficial. Why do I need a new drive shaft if the car shakes just as before? Itís not like I day dream about a shinny drive shaft! Do the right thing. Itís shameful to mislead someone who trusted you with their car, money, and time. You have a shop, and want good reputation, and claim to be a professional car mechanic. Guess what? Doing the right thing is part of being professional. BMW owners buy BMWs because they are passionate about the brand, and not because they are millionaires. We work hard for our money too, you know. We wake up early in the morning, and get stuck in rush hour too, and have to deal with difficult bosses and co-workers, and earn our money 1 dollar at a time, and have to pay the bills too.