What is a Car Diagnostic Test?

Dustin Hawley | Feb 26, 2021

The vast majority of modern vehicles are more computerized than ever before, and truth be told, that’s a good thing. In fact, this computerization allows cars to be thoroughly checked with self-running diagnostic tests to keep your vehicle in optimal working order. These tests can scan components and a car’s integrated systems for errors or malfunctions before they become truly dangerous.

what is a car diagnostic test

Let’s break down what car diagnostic tests are and examine their various benefits so you know when and why you should ask for them at your next maintenance appointment.

Car Diagnostic Test Overview

A car diagnostic test is a digital analysis of your car’s various computer systems and components. Modern vehicles are much more digitized than people may be aware of. Specialized software works whenever your car’s engine is powered on to monitor various features and create data reports that can then be collected and analyzed during a car diagnostic test.

Car diagnostic tests scan your car’s components and systems to check for issues with components like the engine, transmission, oil tank, throttle, and many more.

Because car diagnostic tests require specific devices and expertise to read correctly, most tests are performed with mechanics or at dealer shops. However, you can also perform a car diagnostic test at home if you have the proper knowledge and equipment.

Car diagnostic tests are typically carried out when a vehicle’s dashboard shows a “check engine” signal or other warning light illumination. They can also be performed as part of regular maintenance appointments.

What Parts of a Car Are Tested?

These days, car diagnostic tests analyze various aspects of your vehicle. Specifically, they check for:

  • Problems with your car’s engine or individual components
  • Issues with your car’s transmission and responsiveness
  • Problems with brake responsiveness
  • Potential contamination or faults with your car’s exhaust system
  • Signs of wear and tear or breakage with major components, such as the fuel injector, ignition coils, and throttle

Though these results can help drivers assess their vehicle’s state, keep in mind that car diagnostic tests are not perfectly accurate. They cannot tell the technician or tester precisely what the problem is in many cases. Instead, they’re used to narrow down the location of an issue or potential error so that mechanics can more quickly identify and take care of a problem for drivers. 

How Often Are Car Diagnostic Tests Needed?

In most cases, drivers wait until they take their vehicles to a mechanic or auto body shop to perform a diagnostic. Many other drivers will wait until there is a malfunction or the “check engine” light flashes before they finally concede and take their car to their local mechanic.

Technically, car diagnostic tests are never “needed” in the traditional sense of the word. They’re simply tools that mechanics can use to accelerate the repair process by assessing issues quickly and efficiently. However, it may be a good idea to learn how to perform a simple diagnostic test yourself. With the right equipment, you can regularly test your car for issues and prevent any serious malfunctions from developing over time.

Should you decide to do this, it’s a good rule of thumb to perform a car diagnostic test once every season (quarterly).

How Long Does It Take to Do a Car Diagnostic Test?

Regular car diagnostic tests are performed with a basic scanner and diagnostic software, and they often take just a few minutes to complete. But merely carrying out the test is just the beginning of the maintenance process. After receiving information from the test, repairs or preventative measures must be considered and/or completed.

A car diagnostic test procedure is often carried out as follows:

  • The test begins as the scanner examines the “check engine” light code. Your car’s computer produces a code alongside the check engine light that can indicate the source of a potential issue.
  • Mechanics carrying out the test will typically plug a diagnostic scanner into the car to read these trouble codes and translate them into actionable information.
  • Next, mechanics carry out investigative work to see where and what the problem might be. They may use the error codes mentioned above to narrow down their search area. For instance, the check engine light code might indicate that there’s a problem with the exhaust, so the exhaust will be the first area a mechanic will check
  • After identifying any issues, a mechanic may decide to perform repairs, such as replacing damaged parts or cleaning dirty areas.
  • The test may be repeated after repairs are finished to ensure it fixed the initial error.

How Much Does It Cost?

Car diagnostic costs vary broadly based on your vehicle’s make and model and any potential issues the car may have. Basic tests that don’t turn up any significant issues can be as affordable as $40. However, some more in-depth analyses and their associated repairs could cost up to several hundred dollars, depending on the issue’s severity and complexity.

Furthermore, you should double-check the fine print in any mechanic’s service agreement before signing off on a car diagnostic test. They may claim that they will provide free diagnostic services but can sometimes camouflage hidden fees into the granular details of their service agreements. Run through the print with a fine-toothed comb before agreeing to any services, which will help to ensure that you know all the details. 

Benefits of Car Diagnostic Tests

Car diagnostic tests are valuable for a variety of reasons.

  • Car diagnostic tests help you detect errors before they become catastrophic, saving you money in the long run.
  • Furthermore, catching errors before they become more serious could potentially save your life or the lives of others. Car diagnostic tests can, for instance, tell you when you need to replace your brakes before they fail on the freeway.
  • Car diagnostic tests can also check your car’s onboard computer system for any manufacturer notifications or stored information. The data can help technicians provide the best repairs possible for your vehicle.


Ultimately, car diagnostic tests are beneficial tools that should be a regular practice of any maintenance checkup for your car. To avoid the inconvenience of taking your vehicle to a mechanic, consider purchasing the necessary equipment and carrying out your own regular diagnostic tests to ensure that your vehicle runs as well as possible. A car diagnostic test will help to ensure that your car doesn’t suffer from long-term or catastrophic damage due to malfunctioning parts or computer errors.

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