What Is a Car Diagnostic Test?

[See All Shopping Guides](/cars/shopping-guides) # What Is a Car Diagnostic Test? Matt Yantakosol| Jan 31, 2025 Car diagnostic tests scan a car’s components and integrated systems for errors or malfunctions before they become dangerous. These tests leverage computerization to keep your car in optimal working order and can be executed by a mechanic or as a DIY. Let’s break down what car diagnostic tests are, why they are beneficial, and when they should be conducted. ## **Car Diagnostic Test Overview** A car diagnostic test is a digital analysis of your car’s computer systems and components. The test scans components and systems to check for issues with the engine, transmission, oil tank, throttle, and more. In the test, specialized software monitors features and creates data reports that can be collected and analyzed. Because car diagnostic tests require specific devices and expertise to read correctly, most tests are performed by mechanics. However, you can also perform a car diagnostic test at home. 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 preventatively as part of regular maintenance. ## **What Parts of a Car Are Tested?** Car diagnostic tests analyze several aspects of your vehicle including: * Engine * Transmission * Brake responsiveness * Exhaust system * Fuel Injector * Ignition coils * Throttle, and more Although these results can help drivers assess their vehicle’s state, car diagnostic tests are not perfectly accurate. In many cases, they cannot tell the technician or tester the exact problem. Instead, they’re used to narrow down the location of an issue or potential error so it can be more quickly identified and resolved. ## **How Often Are Car Diagnostic Tests Needed?** Technically, car diagnostic tests are never “needed” — they are simply tools that accelerate the repair process by assessing issues quickly and efficiently. As a result, many drivers wait until a malfunction occurs or the “check engine” light flashes before taking their car to a mechanic. However, you may consider learning 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. 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 often take only 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 plug a diagnostic scanner into the car to read these trouble codes and translate them into actionable information. * Next, mechanics investigate the source of the issue and use the error codes as a guide to narrow their search area. For instance, if the check engine light code indicates there’s a problem with the exhaust, then that’s the first area the mechanic will check. * After identifying any issues, the mechanic will recommend a solution, such as replacing damaged parts or cleaning dirty areas. * The test may be repeated after repairs are finished to ensure the initial error is resolved. ## **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 start at about $40. However, more in-depth analyses and their associated repairs could cost up to several hundred dollars, depending on the issue’s severity and complexity. Furthermore, 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, to ensure you know all the details. ## **How to Conduct Your Own Diagnostic Test** While you may hire a mechanic to run the diagnostic test on your vehicle, you can [conduct the test yourself](https://www.jdpower.com/cars/shopping-guides/how-to-read-your-onboard-diagnostics). The OBD (onboard diagnostic) computer comprises multiple sensors to diagnose issues, which it labels in a 5-character code known as the OBD or OBD-II code. To read the code, acquire an OBD scanner that connects to the vehicle and displays error codes on its screen. To begin the process, read your owner’s manual to locate your vehicle’s OBD port which is also known as the diagnostic link connector (DLC). Ensure your vehicle is turned off and insert the scanner’s 16-pin trapezoidal plug into the OBD port. Insert your key in the ignition and turn it until the power is on, but not the engine. At this point, the scanner’s screen may acknowledge that it is connecting to the OBD computer. Depending on your OBD scanner, you may need to manually enter the vehicle identification number (VIN) or the make and model to proceed. Once connected, look for your codes in the “Codes” or “Trouble Codes” menu. Most devices display active and pending OBD codes. “Active” indicates an issue that triggers the Check Engine light, while “pending” denotes an error. The next step is to decipher the 5-character OBD code, which you can look up [here](https://www.jdpower.com/cars/shopping-guides/how-to-read-your-onboard-diagnostics). * The first character indicates the location of the issue: P (powertrain), C (chassis, e.g. brakes), B (body, e.g. airbags), or U (communication system). * The second character denotes if the OBD code is generic or manufacturer-specific. * The third character narrows the location of the issue. For instance, a 3 signifies an ignition issue. * The fourth and fifth characters zero in on the exact problem. ## **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. For example, car diagnostic tests can tell you when brake replacement is needed 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. ## **Summary** Ultimately, car diagnostic tests are beneficial tools that should be a regular practice in any maintenance checkup. To avoid the inconvenience of taking your vehicle to a mechanic, consider purchasing the necessary equipment and conducting regular diagnostic tests on your own. A car diagnostic test helps avoid long-term or catastrophic damage due to malfunctioning parts. *Matt Yantakosol is a lifelong automotive enthusiast and former consultant focused on the consumer aspect of the industry. He has traveled to New York, Detroit, Chicago, L.A., Paris, Geneva, Frankfurt, and Tokyo Auto Shows to track and observe the newest industry innovations. 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