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What Are Vehicle Diagnostics?

Since 1996, all new vehicles have been equipped with technology to aid in vehicle diagnostics.
Mechanics can test a vehicle's battery as part of a diagnostics test.
Mechanics can explain to customers what the vehicle diagnostic codes mean, and how they should be repaired.
Mechanics conduct vehicle diagnostics to identify and assess vehicle problems.
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  • Written By: Mike Howells
  • Edited By: Michelle Arevalo
  • Last Modified Date: 15 September 2014
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Vehicle diagnostics are the mechanics involved with identifying and assessing problems that may negatively affect the normal operation of a vehicle. Mechanics may employ a wide range of techniques and tools in conducting vehicle diagnostics, ranging from cursory physical checks to more involved, computer-based analysis. Given the sometimes occult nature of mechanical problems, effective diagnostics are critical to successful repairs and the smooth running of any vehicle.

Historically, effective vehicle diagnostics relied on the individual competence of a mechanic, in his being able to use visual or audible clues to correctly assess a vehicle malfunction. Symptoms of a mechanical problem can range from the obvious, such as oil leaking from a faulty seal, to the indirect, such as rough engine idling. Prior to the integration of computer systems in vehicles, mechanics often had to be equal parts detective and surgeon in putting together clues to diagnose and fix problems.

With the advent of electronic control units (ECUs) in cars, airplanes, boats, and motorcycles, however, the task of vehicle diagnostics has become markedly simpler. Through the use of ECUs, technicians can take advantage of computer technology to aid them in identifying and resolving malfunctions. Virtually all modern road vehicles now come equipped with onboard diagnostics (OBD) ports, which work with the ECU to monitor various sensors throughout the vehicle, and log codes when errors or problems are encountered.

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Tied to a vehicle's dashboard or information screen, an ECU can alert the operator when it detects a malfunction by displaying a check engine or malfunction indicator light (MIL) warning. Anyone with a special cable and laptop can literally plug into a vehicle's OBD port and download the error codes to determine exactly what is wrong. OBD ports have greatly expanded the scope of who can perform basic vehicle diagnostics, and many general and automotive parts stores now charge a nominal fee to scan an OBD port and interpret the error codes.

Knowing what is wrong, however, is still only half the battle. An experienced mechanic is still crucial to resolving malfunctions, and there are still many mechanical problems that can occur without triggering an error log by the ECU. Vague rattles and other noises, inexplicable loss in fuel efficiency, and steering that pulls to one side or another are all examples of operator complaints that may not be recorded by the computer, but can be further investigated and successfully resolved by a skilled professional.

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Discuss this Article

strawCake
Post 3

@KaBoom - I sort of think that if you don't know about cars, it's not really going to help you to know why your check engine light is on. For example, it may be on for one problem. But it's possible for one problem in a car to create another problem, you know?

So it's totally possible for a mechanic to tell you that there are more problems than just the one causing the check engine light. I'd hate for you (or anyone else) to avoid getting something necessary done to their car just because they don't trust their mechanic! Just find a good shop that you can trust and stick with them if you don't plan on becoming more knowledgeable about cars.

KaBoom
Post 2

I really like using vehicle diagnostics when my check engine light comes on. I don't perform the repairs myself, but I like to be informed when I take my car to the mechanic.

I find that all too often, mechanics try to get you to have extra work done that your car doesn't really need. At least if you have some idea of what's wrong you'll be able to tell if your mechanic is telling the truth or not!

ElizaBennett
Post 1

If your check engine light comes on, a lot of mass merchandisers like AutoZone will do a free automotive diagnostic for you. (Sometimes there is a charge if you do not have a repair performed.)

But it seems like it can be frustratingly inconclusive. Sometimes the machine that hooks up to your car to talk to it will record a whole bunch of different codes, and it takes time for the mechanics to sort through which one or ones are the "real" problem. Or the light comes on and the machine still isn't sure!

It will always take a skilled mechanic to interpret what those machines are saying, not to mention actually doing the repair.

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