"Patience" was one of my Nonno's (Grandpa's) sayings. Yes, he was a man of few words but what he did say was probably worth taking heed to. Our current society does not practice "patience". We are into "right now".
I guess we do not show much patience in life because we do not often need to be very patient when we want something. We are a society of fast food and fast Internet.
In my trade, speed is important. The old adage "time is money" comes into play frequently. Sometimes though, a little extra time will fix a car that otherwise would not be fixed. There is nothing I like more than a fixed car.
Many customers have a very simplistic view of auto repair. The car does this and then the mechanic does that. Voila! Fixed! How could it cost so much? These people get very nervous when their vehicle is not in and out in a day. Another day and their confidence in their mechanic is waning.
When a vehicle has a problem and it is intermittent or a bit of a challenge, I sure appreciate a customer that is patient. Sometimes a vehicle has to be driven to produce the symptom and sometimes each drive does not produce the symptom. In these cases driving the vehicle all day long is not possible nor a cost effective means of diagnosis.
Symptoms usually require conditions which may not be easily reproducible. In fact, they may not even be known. The problem as described by the customer has to be experienced at least one time before the mechanic might even know where to begin.
Once experienced, the symptom might have a simple solution but sometimes it does not. Most symptoms have multiple possible solutions. Problem; the vehicle stalls while idling. Solution; turn the idle screw up. Not likely. Turning screws only causes more problems for the real mechanic to correct after the "screw turner" has worked his/her wisdom on it.
The testing procedure must now be formulated. Most likely some disassembly and instrumentation will be involved. Having a patient customer who lets you keep the vehicle overnight with all instruments connected up will be greatly appreciated. The vehicle is then available for testing anytime the mechanic has an opportunity. Each test drive may require reproducing a particular state of circumstances.
Vehicles misbehave when they are cold, when they are hot, when they are climbing a hill, etc. A cold start problem may only allow one test per day after sitting overnight. There has to be other productive work scheduled at the shop while waiting for the problem child vehicle to meet its next testing state conditions.
A scan tool set to record specific input and output data will typically be used during initial testing. Once information is gathered a more detailed test procedure can follow. It is very common now that your mechanic will be using a digital storage oscilloscope to collect data from the vehicle sensor's output devices and testing sensors all at once.
Hopefully the next test drive produces that "eureka!" moment when the culprit part is found. Patience though! Another test drive might be required.