Unless the part(s) are under warranty, I always leave them with the client. I have replaced a few glow plugs. After the furnace is back up and running, I always show the client where the plug failed. In all cases, i show the client the problem. I did this when i did TV repair in the 80's (I was in my teens). To me, showing the client the problem and why I did what I did is the most important thing.
I run a small auto repair shop with a couple friends and we make a habit of doing the same thing. Doesnt matter how big or small the part is we keep it to give the customer the option of seeing it. I find that people are more willing to pay the bill, especially the diagnostic fee, if they can hold the source of the problem in their hand. Plus theres no way they can say that you 'didnt do anything' . Especially it the part is hidden in the engine. Even if its just a tuneup, i keep every part till the customer says throw it out.
And from my end on the electronics repair end of it, i brought a TV in for service not too long ago and the small, out-of-the-way-hidden-down-a-one way-road repair shop kept the defective part for my TV (cracked HV boot arcing) to show me exaty what failed. Let me tell you i will definatly go back to them again because they cared enough about my satisfaction to do that for me!
Sorry its off topic, i just had to add my comment
