I educate my customers on the job I do, and hand them their old parts too. The trait you mention is usually very prevailent in low educated people. Some people complain that you changed the brakes and now my rear wiper blade rubber has rotted away - its your fault.
Those are morons and should not breathe the same air as other animals. And I do not entertain those people even for a minute - I refuse their initial jobs too as Im pretty good at identifying them too.
My way of work is to get machine up to "service spec" and then follow a schedule of maintenance as the manufacturer wants. e.g. if I changed ball joints on a corolla which now have greasable type, This job is now added to the maintenance schedule which is regrease with 2 pumps every year - or if car drove in deep water. - If I find wiring unwrapping near my working area, I rewrap it, If I see a bolt is damaged I replace it, If I see rubber trims falling off I replace their plastic clips (have all sorts of them) - If I know some problematic cases like bad contacts - I apply silver paste to them on my own choice (no need to bill extra).
Customers who call me to ask - "when should I visit you" I ask them - if maintenance schedule says so or you have a failure? - they say no - I say keep on driving (I actually tell them to not come) - They are satisfied since years now.
now coming back to diesel lab person - 95% of car owners in Pakistan are clueless about diesel engine working principle. 99% mechanics have no idea of fuelling - thats why scrappers are able to sell 1KZ with 2C diesel pumps on them and claim that "engine start hai"
the old adage of "andho mein kaana raaja" is what is happening, - while with just a little knowledge and some effort you can find a qualified 20/20 vision "raaja" to work on the equipment.
Just like - people visiting quack doctors also have 95% customer satisfaction rating. Doesnt make those quacks real qualified doctors.
so back to point - unless you can "measure" performance and quality of a job - any job may look excellent - in plain urdu - "peetal aur sonay ka farq pata hona chahiye".
The reason I wrote that I would see the fuelling of the pump at DPS/Cummins - is that I am eliminating all errors on repair of pump - They showed me the values of manufacturer and their work which matched - this means their job is top notch. Now if any anomaly occurs Its error of installing - This is not an alien concept - you should be able to account for your work from cradle to grave - its called project management.
Just like electrician repairing alternator - and your vehicle wiring is bad, How would you deduce whose at fault?