Diagnostics... Where to start?... as it is constantly evolving. Let's start where most people unfortunately start: the dreaded check engine light that sometimes comes with notable symptoms in vehicle performance (or lack there of). Nobody likes seeing that light. Why would you?
Well, after working in the automotive industry for over a decade I have seen this little light so many times I see it embedded in my eyes when I close them to go to sleep. Okay, that's a stretch, but you get the point. There is a lot of expectation that customers get when they bring a vehicle in with a "check engine light" activated. Although it is a sign of a code being stored, it should be noted that simply plugging into a vehicle with a scanner and getting the code is far from all that's needed for a mechanic to know what is going on. Codes work like notable bread crumbs in a diagnostic trail. There are hundreds of different Codes, many of which can be thrown by different symptoms, combinations of parts or parameters to cause a code to be activated.
What should be said before moving further into code scanning, is that vehicle monitoring systems work on rhythms and signals of electronic data. Wave forms and constant monitoring of electronic data. Vehicles have computer processors that have set parameters of what optimum performance should look like in data streams. When performance is thrown out of that tolerance, it can pinpoint what and where, but not how.... This is where diagnostics starts and also where most customers understanding of what technicians do unfortunately ends.
Let's take a generic misfire code, for instance. Usually accompanied with a vibration at idle, sluggish performance, a drop in fuel economy and peak power, a generic misfire code can be one of the most time consuming codes to chase, simply because the code states that a cylinder has skipped it's firing cycle, even worse if it is a random misfire. Now comes the daunting task of understanding why. It's usually where a lot of science and large words start coming into play and tensions begin to rise, as do costs. I guess I empathize for both parties in this case, so I am attempting to shed light on the technicians' side of this situation as often it comes from a disgruntled customer who feels they got jerked around and has lost confidence in their shop due to a potential misunderstanding.
So, back to the example of our generic misfire code you just pulled. Consumer feels, "Awesome, you got the code. What's wrong and how long?" Well... that's a loaded question a technician is unprepared to fully answer at that time. So, let's look at what is known at the time. We know the poor performance is due to a cylinder not firing properly. So we go to the science: What causes a cylinder to fire properly? First, you have compression in the cylinder which takes a proper amount of fuel and air mixture and it compresses that mixture inside the cylinder toward the spark plug at the cylinder head. This causes heat due to pressure, making the fuel air mixture easier to ignite when the electrical arc of the spark plug fires. Already, as you can tell, there are many factors at play in making a controlled explosion inside a cylinder that happens once every two turns of the crankshaft. Meaning at an idle of 900rpm, that cylinder is going through an explosion 450 times each minute. Anyways, back to the diagnosing of said cylinder not "firing." Technicians have to first consider or identify which element of the equation was removed—compression, proper air, proper fuel, and/or proper ignition. All come with many potential possibilities. So it's a process of elimination and experience helps technicians cross out potential causes faster. Sometimes it's just dumb luck and you get it right the first time.
The fact is, dishonest mechanics are fewer and farther between than mainstream ideology paints a picture of. In total transparency from inside the industry, most technicians I have met or worked with are good people, honest and smart workers just trying to stay on top of a constantly evolving and challenging industry. The industry is troublesome enough given ever advancing technology and demand from employers to understand and perform at peak efficiency, let alone working around dishonest practices and lying.
Not to discredit some people who have legitimately met the few dishonest techs that give any honest tech a bad name. Truthfully, I believe most mechanics hate shady technicians more that consumers do, simply because of the stigma it creates with clients long before we attempt to work on anyone's vehicle. Either way, I am just stating that they aren't as common as people believe.
Okay, back on track and back to work. So we have to start ruling out potential causes. It's best to work from the source outward, best to go to where compression happens. Let's remove a coil pack or plug wire and take a spark plug out. Spark plugs tell stories about chemical combustion based on their condition and discoloration. It can rule out the absence or presence of chemicals and/or long-term running conditions. There is no time to run through all the potential variables, so for argument's sake we will say that the color and condition points toward lean combustion (more air than fuel in terms of optimum ratio). It should be noted that air to fuel mix in gasoline engines is supposed to be a 14.7:1 ratio already. So... where is the extra air coming from? It's possible that more air is getting in, or not enough fuel getting delivered. Too much air could indicate Mass air Flow reading issues, vacuum leaks, throttle body performance problems, EGR (Exhaust Gas Recirculation) system problems, to name a few potential causes.
Not enough fuel... could indicate injector issues or a blockage in the fuel rail. If the misfire is on multiple cylinders, then it could indicate fuel delivery further down the line like pressure issues, or a bad filter or pump. Again, there are many variables to consider.
So we're going to say we believe it is caused by too much air as fuel delivery seems apparent in our experienced opinion in this example. Vacuum leaks are common in many vehicles, especially when air delivery systems are constantly evolving in modern cars to improve fuel economy and emissions. More pieces in a system equals more chances for faults right? Unfortunately, yes, new technology has its downsides often in automotive science, as well. The old saying "they just don't build them like they use to" applies heavily to that exact fact.
Many people off the bat would assume air goes into a motor from point A to B. It makes sense. Air goes into a tube and out the other end. Unfortunately, that is a wrong assumption. Modern technology requires that it goes through multiple tubes, plenum structures and sensors, and undergoes necessary heating and cooling and in some cases spoiling and pressurizing systems before being mixed with fuel. Due to EGR systems, that air is also made up of 10-20 percent emission vapor from previous burned mixture being rerouted from the exhaust to be re-burned to make cleaner emissions. The amount of science, math, and chemistry involved in this process, like many that happen in microseconds inside of a car, is why technicians undergo so much school. It may look like we bang on metal, turn wrenches, and swear at inanimate objects... Well, we do that too, but it is far more in depth than it appears from the outside.
So, in this theoretical run-around, we are chasing a vacuum leak and begin testing seals along the intake system to find that the gasket around the intake manifold is worn and air is getting sucked in. Well, bingo! We have a cause. In a perfect world, it only took an hour or less to find that out. Because regardless of the time it took to track that needle in a haystack down, you now have to explain to someone still at square one who just knows there was a misfire code.... that you have to remove multiple components, spend hours of labor time to replace a part that can be anywhere from 35-300 dollars depending on the type, make and qualities of the part, as well.
This is where many customers lose patience and confidence. Delivery of cost and findings in diagnostics is a very delicate art. Firstly, explaining the route and the science is not always easy. It takes a certain amount of large words and also a delicate delivery of those. Nonetheless, regardless of whether the customer understands what you are saying, the financial pill to swallow is often not easy. And the "guilty until proven innocent" stigma carried by most customers to begin with never makes the situation easier.
The potentially disgruntled customer is wondering, "How is it so much?" "How can you be sure I just don't need an oil change?" or just plain "Are you serious?!" Well, in many cases when diagnostics has been done properly, no amount of anger or frustration is going to change the outcome by much. You are either left to continue with the problem if possible or unfortunately fix the issue. Some alleviations can be to source used or rebuilt parts to save on cost, or get someone you know and trust that is mechanically inclined to do the work for a case of beer....
In my opinion, and I say this from seeing numerous failed attempts... one must be careful in having a buddy fix car problems for a case of beer. And not because of the beer. Best case, the issue is solved, but there is a lot of margin for error. Many terrible things start with good intentions and vehicle repairs are no different. Consider risks of potential failure. Not just wasted time and money... and beer. But it can put drivers and others at risk of potential injury or death if something goes wrong with the wrong repair.
I don't mean to discourage people entirely as I have seen many handy people exceed at repairing cars without proper training, tools, and even proper parts. But money isnt everything and as technology advances, "farm fixes" and backyard repairs are becoming less and less feasible. My personal opinion is to cut cost on the parts by searching for used or refurbished parts and have a trusted and qualified person put them in at a fair rate, with peace of mind to comfort you for your extra money spent.
This story is meant to shed light on the process, as well as the other side of the story from technical perspective in the matter of repair itself. It is by no means a thread to diagnosing any misfire or check engine light code as there are, again, hundreds. But if you are unfortunate enough to have to visit the repair shop, as everyone who drives eventually will, maybe consider that your technician is actually smarter than they appear, is honest and takes pride in their work, and wants your trust and patience.