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Why Does Diagnosis Matter (And Why You Can't "Just Scan" A Car)

Diagnosis is one of the few things that (in our opinion) separates mechanics from technicians. While it might be a subtle difference, for a lot of vehicle owners this skill can also make all the difference between having a good experience and spending $$$ and not having a fixed car.

But, isn't diagnosis just plugging in a computer and replacing what it says?

Well, not really. The act of diagnosis is "the identification of the nature of a problem by examination of the symptoms" and, while reading the trouble codes from the computer is a good first step, it goes exactly nowhere in fulfilling the definition.

So, how does a mechanic think about diagnosis when compared to a technician?

The first main difference is that mechanics tend to look at diagnosis as the process of eliminating everything that is good on a car. While a technician or a home-gamer might look at a coil-misfire code and try to replace a coil pack, a mechanic will check the operation of each system that can effect that until there all possible failures are exhausted.

Taking a misfire code as a good example (this is when an engine doesn't fire on one or multiple cylinders), a mechanic will check for the presence of air, fuel and spark first, then for an individual component failure in any of these systems.

The second (and far less crucial difference) between a technician and a mechanic is the ability to quickly eliminate potential problems and move onto the source. This is actually a skill learned with time but makes a huge difference for the success of diagnosis.

Using quick tests and other signals from the car to narrow the search to the right area quickly can make a huge difference. Such as hearing a clicking noise from the start motor on a no start diagnosis or doing a coil disconnect test on a misfire.

While the end result might be the same 50% of the time, when you're dealing with other peoples money and property, there's little room for mistaking a diagnosis (and spending extra money).

Some of the really good workshops don't actually think about success as the amount of cars fixed but the amount of cars fixed on the first try.

So why does this matter for vehicle owners in particular?

Diagnosis might seem like an unnecessary expense when weighed against advice from the internet and a basic code scan but, there have been a number of times that we've seen directly where proper diagnosis has saved thousands of dollars in repairs that would have otherwise been given the go ahead.


If you have any questions about this blog post, would like a second opinion from a mechanic or would like to find an honest mechanic in Brisbane, check out Kashy here.

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