What is a Car Diagnostic Test? Everything You Need to Know

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man performing engine diagnostics

As the innovation and technology in automobiles continue to grow, so do auto repair technicians' diagnostic tools and methods. Many modern vehicles are now equipped with an onboard diagnostic system that allows technicians to plug into the car and run diagnostic tests.

But what exactly is a diagnostic test, and how is it performed? Whether you are a car owner or planning to have your car in the future, it is crucial to understand the basics of diagnostic testing. So, stay tuned and read on! 

What is a Car Diagnostic Test?

A diagnostic test examines your vehicle's systems and components to identify any potential issues or problems. The test will usually start by reading the trouble codes that have been stored in the car's computer system. 

These diagnostic trouble codes are diagnostic "flags" that indicate something is wrong with the vehicle. Once the diagnostic test identifies which codes are present, the mechanic can then narrow down the possible causes of the problem.

How is a Diagnostic Test Performed?

A diagnostic test is usually performed with a special tool called a diagnostic scanner. This scanner is connected to the car's computer system and allows the mechanic to read the trouble codes. 

When the scanner retrieves any trouble codes, the mechanic will save them in a document form for further diagnosis. For you to be able to understand it more precise we make an example below:

Your car has drivability issues, and you decided to bring your vehicle to a check-up. The mechanic will get an OBD scanner and plug it into your car's diagnostic port. If he gets the error code triggering the Check Engine Light, he will scan for other trouble codes that may be causing your car problem. The mechanic will also perform the following:

Visual Inspection

This is one of the most important diagnostic steps; the mechanic will check for any physical problems with your car, including:

  • Fluid levels
  • Belts and hoses
  • Leaks
  • Air filter

Major Components Testing

After the visual inspection, the mechanic might test some of your car's systems depending on the diagnostic trouble code and the exact issue. However, these are some of the common tests when a diagnostic trouble code is found:

  • Fuel System Test- The mechanic will check the fuel pressure and composition to ensure your car gets                                              the right amount of fuel.
  • Engine Brakes- The mechanic will check the car's electronic brake system to find any issues.
  • Transmission- The mechanic will test the transmission by shifting through the gears and inspecting the                                      pressure.
  • Heating and Cooling System- The mechanic will ensure the car's cooling system is working correctly.
  • Spark Plug Test- The mechanic will check the spark plugs to see if they are firing correctly.
  • Compression Test- The mechanic will check the fuel pressure and see any leaks in the system.
  • Ignition System Test- The mechanic will test the spark plugs and wires to see if they are working                                                               properly.
  • Exhaust System Test- The mechanic will check the oxygen sensor and catalytic converter to see if they                                                   are working properly.

Repairing/ Replacing of Faulty Component

Finally, when the mechanic finds out the exact issue of the vehicle, he will now proceed with the repair or replacement of the faulty component. This might involve something as simple as replacing a spark plug or something more serious like repairing the engine. The mechanic will also test the vehicle again after the repair to ensure it runs properly.

How Long Does a Car diagnostic Tests Take?

Not all diagnostic tests are the same; it isn't easy to estimate how long each diagnostic test will take. For example, a diagnostic test on a car's engine would take much longer than one for the brakes. However, most diagnostic tests usually last no more than an hour.

What Should I Diagnose?

When we say diagnostic test, we're not just referring to one test but rather a series of tests. These tests are done to figure out what might be wrong with your car. Here are some of the car parts that you should diagnose.

Engine Diagnostic Test

This is one of the most important diagnostic tests on your car. The engine diagnostic test will help identify any issues with the engine itself and any other components that might be causing problems, including the fuel system, the ignition system, and more.

Transmission Diagnostic Test

This diagnostic test will help identify any issues with your car's transmission. If there are problems with the transmission, it can cause various other problems, including engine stalling and even complete failure. The components that will be tested associated with the transmissions are the torque converter, the clutch, and more.

Brakes Diagnostic Test

The brakes diagnostic test is essential to your car to ensure that the brakes are in good working order. This diagnostic test will help identify any issues with the brake pads, the brake rotors, and even the brake fluid levels. All of these components are essential for proper braking function.

When Do I Need to Perform the Diagnostic Test?

It is essential to let your car get a diagnostic test done at least every six months. People will often think about getting one done when a problem is already present. We would disagree with this notion, as preventing any issue is way better than having it fixed when it just happened. 

Diagnostic tests are part of car maintenance and are designed to help you fix problems even before they have a chance to become more expensive and more of a hassle to handle. 

Think of it this way - would you rather spend a few hundred dollars on a diagnostic test and catch the problem right away or wait until that same problem has cost you thousands of dollars to fix?

The choice is obvious! But since most of us are born stubborn and won't take advice most of the time, take your car for diagnostic at least when:

  • Your Check Engine Light illuminates
  • Your vehicle is having problems, and you can't figure out what's wrong
  • Your vehicle isn't performing as well as it used to
  • You're getting ready to sell your car
  • Your vehicle is scheduled for an emission test

What are the Benefits of Performing Regular Diagnostic Tests?

Like a human body that needs a regular check-up to ensure good health, a car also needs to be checked and diagnosed regularly for any problems. This ensures that any pre-existing issues are found and fixed before they become bigger and more expensive to repair. Aside from these diagnostic tests, you can:

  • Prevent costly repairs down the road
  • Get an understanding of your car's condition
  • Keep your warranty valid


A car diagnostic test is essential to keep your car in good condition. It is a preventative measure that can save you money and time in the long run. It may sound like a daunting task, but you can even do it yourself with the help of accurate information and guidance.

But as we've always said, bring your car to a professional mechanic if you're uncomfortable with the task. It's better to be safe than sorry! What's your experience with diagnostic tests? Let us know in the comments below!