duo
sometimes in troubleshooting trees, partial or complete teardowns are necessary to facilitate testing on individual components
for example a toyota FBC with a code "C0"
at some point in the tree you have to take out the card, all the connections, the busses on the IGBTs' that put them in parallel, remove each from the heatsink, and test individually
but they may be good and all that was for naught
if one or more are bad you have to check every power connection in the truck for voltage drop including the drive motor still not find anything and have to monitor operator driving habits for abuse as those IGBT's are sensitive to overloading

so to say you know exactly what is wrong before testing was done is presumptuous and foolish and personally i hate when ppl give time frames b4 ive had a chance to diagnose a single wire
in MY experience
techs know the right way to test and replace components
the other guys "playing mechanic" are out there basically guessing for the most part
weed out the parts swappers and youll watch profits go through the roof GPS or not

EVEN WORSE is when some1 IN management goes out,runs into a customer with an issue, and says "o yeah thats not a problem justin can come fix this in five minutes"
then i get there and i have to inform the guy
"no im sorry but the diagnostics will take roughly 1-2 hours and if its what i think it is its gonna be another 2-3 hours to fix it"

how is that my fault if the customer goes nuts after that?

remember if youre mad about the people you have out representing your company YOU hired them!
its still managements fault for not finding, hiring, and training qualified techs
if they make a mistake WHOS responsibility was it to ensure that person knew what he/she is doing before they were even dispatched????
dont give every1 who fills an app out a set of keys to a van