What Is a Car Diagnostic Test?

What Is a Car Diagnostic Test?

“Back in my day,” mutters your uncle, before launching into a story about how mechanics used to diagnose cars by smell, or fix engine issues with a hammer and a can of whale oil. Fortunately, the days of peering over an engine bay searching for clues are long gone, thanks to on-board diagnostics. But what can a car diagnostic test tell you, and how does it work?

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What is an engine diagnostic test?

A car diagnostic test is a maintenance and analysis system used by a mechanic to find out what your vehicle’s computer thinks is wrong. When you see that alarming “Check Engine” light on your dash gauges, you’re looking at a part of the on-board diagnostic system. Over the decades, what started as just an engine diagnostic for emissions systems turned into far more. Now, computers have taken over subsystems in vehicles, such as navigation, advanced all-wheel drive, active handling systems, active safety features and more.

In the early days of electronic fuel injection, each manufacturer used a different system for writing code into their vehicle’s simple computer systems. When the engine was running, the computer checked a few vehicle sensor readings against stored parameters and adjusted the spark or fuel as needed for increased efficiency. This was the first era of on-board diagnostic systems, later called “OBD” for short. These early computers were about as high-tech as a dollar store calculator, but they succeeded in lowering emissions and increasing gas mileage over the old carburetor. Unfortunately, the massive amount of computer systems from each manufacturer was a nightmare for mechanics, who understandably didn’t want to buy every manufacturer’s diagnostic software and equipment for several thousand dollars each.

The Society of Automotive Engineers recommended standards for on-board diagnostic systems in order to alleviate problems for mechanics and vehicle owners. This created the OBD-II standard for 1996 vehicles, featuring a universal diagnostic plug located under your dash, and standardized signals or diagnostic language being passed to any brand of OBD-II reading equipment. This standardization of technology created a universal language for diagnostic trouble codes. For example, a car diagnostic test might return a code of “P0420,” which everyone from Ford to Fiat mechanics knows as a common sign of a failing catalytic converter. Standards also significantly lowered the price of equipment. The cost of OBD-II diagnostic equipment has fallen drastically, allowing even part-time do-it-yourself mechanics to use this tech.

How does a car diagnostic test work?

When you see that “Check Engine” light, you might head into a dealership service center or your local mechanic for an engine diagnostic test. The tech connects their diagnostic equipment, which is able to read the code stored in memory. They use this information to further diagnose the car and solve the problem.

For a more detailed example, let’s say you’re driving along in your 1996 or later car. While it is currently operating normally, it has a lot of miles on it, and the oxygen sensor is slowly getting coated with carbon buildup. The vehicle’s engine diagnostic system sees this happening, reading increasingly low sensor output over time, but allows it as long as it falls within normal operating parameters. Once the sensor is too coated with buildup to function properly, the OBD-II system illuminates the “Check Engine” light for you and stores a code “P0153” in memory. After taking your vehicle to a local mechanic, they connect their standard OBD-II diagnostic equipment and power on your vehicle.

The diagnostic equipment communicates with your vehicle, essentially asking what is wrong in computerspeak, and sees the saved code. The mechanic’s advanced equipment may detect that the likely culprit is a faulty oxygen sensor, and even home in on which one if your vehicle has several. The mechanic replaces the oxygen sensor, and then uses their equipment to delete the stored code. Another car diagnostic test is performed to make sure the new sensor is operating properly.

What can a car diagnostic test tell you?

We’ve shown how car diagnostic tests are a handy way to help figure out what is wrong with your vehicle. You’re probably wondering if a car diagnostic test can show everything that is wrong with a car. Unfortunately, that’s not the case. While super useful, engine diagnostics are limited to the various engine sensors they communicate with.

What can a car diagnostic test tell you?

  • It’s time for an oil change.
  • Oil or coolant levels are low.
  • You’ll fail an emissions test, due to a worn-out oxygen sensor.
  • A specific sensor, such as mass air flow or air intake temperature, has failed.
  • Coil pack is starting to fail or has failed.

What a car diagnostic test can’t tell you:

  • Brake pads or rotors are worn out.
  • The incorrect tire size was installed.
  • You’re using the wrong fuel octane.
  • A headlight bulb is burned out.
  • The car needs an alignment.

When should I get a car diagnostic test?

Now you’re probably wondering if you need a car diagnostic test. Generally speaking, if you don’t see signs of a malfunction, you’re probably good. A car diagnostic test isn’t something that needs to be performed regularly or even annually. But once you have a “Check Engine” light or signs of a malfunction such as engine stumbling or poor gas mileage, it’s time to take the car to a shop.

Signs you probably should have a car diagnostic test performed:

  • The “Check Engine” light is on.
  • Other dash gauge warning lights are on or flashing, such as the “ABS” or “Battery” light.
  • The engine has poor performance, stumbles, smokes, backfires or uses too much fuel.
  • You just had new parts installed to correct the condition causing the “Check Engine” light.

You can probably skip the car diagnostic test if:

  • Your vehicle has no warning lights.
  • The vehicle is operating normally.
  • You only had the oil changed.
  • A service tech is pushing it at a high cost, “just in case.”

Can I run my own car diagnostic test?

If you’re the DIY type or you’re looking to save a few bucks on repairs, you can perform a car diagnostic test yourself. You have a few ways of doing this, depending on how much diagnostic capability you need and how much you want to spend.

On the low-cost end, look for free apps for your smartphone, such as “Torque,” “BlueDriver” and “OBDLink.” Then purchase a wireless OBD-II reader that plugs into the diagnostic port. You can find these online for under $20. When you connect the OBD-II reader to your car and open the app, your phone turns into a code reader, able to read diagnostic trouble codes and often giving you expanded info on what is wrong. Keep in mind that not all diagnostic scan tools work with all apps, so check compatibility before buying.

If you already have too many apps cluttering your phone or just want a dedicated tool, there are plenty of highly reviewed code readers in the $30 to $60 range. While these affordable models don’t offer much more than the code reader apps, a separate tool offers the option to always have it on hand, even if your phone is out of battery.

On the other end of the spectrum, professional code readers can easily run over $1,000, offering a large display, numerous languages and in-depth diagnostic capability of subsystems such as airbags and ABS. You probably don’t need this level of code reader unless you want to open your own car repair shop, or really enjoy DIY repairs.

Keep in mind, even the most advanced diagnostic code reader isn’t perfect. The tool can only display what the vehicle’s sensors are displaying. Code readers usually only offer a general area to look at, not a specific part. Even the most advanced code reader won’t tell you “replace leaking fuel injector on cylinder number 6.” Since it’s only a part of the engine diagnostic process, code readers won’t replace professional car mechanics anytime soon.

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At Bumper, we are on a mission to bring vehicle history reports and ownership up to speed with modern times. A vehicle is one of the most expensive purchases you'll likely make, and you deserve to have access to the same tools and information the pros use to make the right decisions.

About Andy Jensen

Andy Jensen is a former reporter, now automotive enthusiast writer. He covers industry news, manufacturing, car reviews, race recaps, maintenance how-tos, and upgrades. Andy has contributed content to Jalopnik, Advance Auto Parts, Carvana, and zeroto60times.com. His project car is a modified Scion FR-S, but he’s probably looking at $400 beaters on Marketplace right now.

Disclaimer: The above is solely intended for informational purposes and in no way constitutes legal advice or specific recommendations.