When it comes to automotive repair, there are several methods of approaching the problem. Let’s look at: known problems first, maintenance as pair, comprehensive versus fix just what is broken, as well as a few diagnostic approaches.
Until we correct all the known problems in a system, we can not know if they are effecting the portion of the system we are concerned about. Often times tracking down the cause of one problem will reveal the cause of others. Use this method with a bit of common sense, replacing a gas cap for an evaporative leak will most likely not fix a misfire. However, often times vacuum leaks cause many, many problems. (For a specific example of fix know problems first, visit here )
Furthermore, the vehicle has limited communication options, just a general “problem” light. Vehicles often come into the shop with numerous problems because the check engine light has been on. Customers are shocked to find they have a bad cat, a failed oxygen sensor, and a misfire, all because they have had the check engine light on (for the bad cat) for so long. If we maintain a vehicle properly, the check engine light will alert us to a problem as soon as it presents itself. By fixing all known problems first, we often cure other problems, or find the root of other problems, and we often prevent future problems.
Many, many times the repairs we perform are actually routine maintenance. A vehicle that comes in for a misfire, due to a worn out spark plug, will receive a full ignition tune up. This repair, is actually just maintenance. If the proper maintenance had been performed, the misfire would have not developed. The rings would have not been washed, the gas mileage would have been preserved, and the customer would not have had to pay for diagnostics. Furthermore, the customer would have been able to plan for the expense.
If you carry this same mentality forward, we may replace a fuel filter knowing it is old, but not sure if that is the root of the problem. If a vehicle has a lean condition, and lower than expected fuel pressure to the engine, the maintenance of replacing and old and clogged fuel filter may actually repair the problem. This maintenance may not fix the problem, but if the fuel filter is old it needs to be replaced. The maintenance is also a repair that is fixing a known problem. I would only suggest maintenance as repair if you simply can not find the cause of a problem and it is your last resort.
Comprehensive repair approaches the vehicle as more of an organic thing. The entire vehicle will be inspected every time it comes into the shop. Often time these facilities will refer to an oil change as a 3,000 mile service. The customer is presented with a laundry list of recommended repairs, much of which are maintenance.
The reason I subscribe to this approach, is that maintenance is always less expensive than repair. As mentioned before, the customer will save by not paying for diagnostics. The customer also saves money because the are avoiding towing the vehicle in, and a problem caused by lack of maintenance will often lead to other more expensive failures. The customer, if not properly informed, can be put off by the long list of suggested repairs. Because the customer can be driven off by the sticker shock of maintenance, many shops just fix what is broken.
Fix what is broken is a popular approach to automotive repair. It takes less work to simply determine what is wrong with a car and fix that rather than provide comprehensive repair. More cars can be turned in a day, and the customer does not have the sticker shock effect that often comes from a comprehensive inspection.
The draw back being that the customer receives a less complete service. Most customers expect that when a vehicle passes through a shop the mechanics will somehow spot every possible flaw in the vehicle. This belief is unreasonable, but is out of the question when the car visits a fix what is broken shop. Furthermore, you’ll often hear a customer ask, “how did they car look?” A fix what is broken shop will inevitably respond “everything looked o.k.”, because they actually have no idea how it looked. Fix what is broken may be easier on the shop, and appear to be less expensive, but in the long run it is a disservice to the customer.
Theories of repair often stem form theories of diagnostics. Let’s consider a few common options to diagnostics.
Once you understand a basic diagnostic loop, you can then isolate each component in the loop and test. The difficulty here is that you must rely on dat provided. Although you measurements may seem to be within expected values, there are often many factors what you can not account for.
One of the most comical approaches to automotive repair is the “Known good part” approach. This theory says that if you replace all the parts in a system, you are bound to fix it. One by one, you replace all components of a system with known good parts. When the problem goes away, it is generally attributed to the last part replaced. The trouble here is that it may have been attributed to a combination of the parts replaced. Not knowing exactly what fixed the issue tends to lead to faulty diagnostics in the future.
The hangs parts approach could be called the “it’s usually this” approach. Basically you pull a code or read a complaint, make a guess, and then order a part and install it. Some “advanced technicians” will first run the code through napafix or identafix to get an idea how many people have replaced what, but in the end they are still just guessing. All-thought it cuts diagnostics time way down, it is rarely successful in the long run.
Although the shotgun and hang parts approaches seem unreasonable, what we must remember is that doing something may fix the car, but doing nothing certainly won’t. If you are unsure about a diagnosis, and the part is inexpensive, order it in and replace the part. If that part fixes the problem, you can then sell the customer on the part. If you have the same result, you can eliminate that part and you will most likely learn something from the event.
In the end, most shops use a mixture of all the repair theories, as they mostly all have their time and place.