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What is a Car Diagnostic Test & How to DIY?

Car diagnostic test


You know your car best. You drive it daily and know what it should look, sound and smell like.

But when something goes wrong, like your check engine light comes on or you hear an unusual sound from underneath the hood,  you may find your car not familiar as it should be. 

That's where a car diagnostic test can come in handy. 


What is a car diagnostic test?

A car diagnostic test can determine if your vehicle has issues with its engine, exhaust, transmission, ignition coils, oil tank, throttle and more.

Using your engine's computer system, the test — generally conducted by a mechanic — will simultaneously check each and every sensor and microchip in the vehicle to determine where the issue is. It will look for error codes, manufacturer notifications and more.


How much does it cost for a diagnostic test?

The cost of a diagnostic test can vary greatly depending on your vehicle's model, make, and year. Experts say you can expect to pay anywhere between $40 and $400. And that's before any repairs are done. You should call at least three different places to get a quote before making an appointment to get a diagnostic test. 

If you search for car diagnostic test online or ask a dealer for quotes, you can see that the diagnostic service usually consists of two steps:

  1. Engine Light Code Pull Service(usually free)

  2. Engine Diagnostic Service(usually costs $40-$400)

 If you really want to save some cash, investing in a vehicle health monitor can help you to do the first step by yourself. Here we will guide you through this process.


How long does a full car diagnostic test take?

The simple answer is several minutes or hours.

A simple car diagnostic test should only take a few minutes to complete. The mechanic will, however, need time to look further into any error codes that appear. And before visiting the auto shop, you should be prepared to wait a little bit before the test and stay afterward to discuss what issues were found and how they can be resolved. 

Depending on what kind of car diagnostic tool the mechanic is using, they will only be able to tell where the problem is by looking at different codes that appear on their diagnostic tool. They will then need to do some investigative work to get a more accurate picture of what is functioning incorrectly and how best to fix it.

Car diagnostic tool

How do I run a car diagnostic?

Though it sounds complex, a car diagnostic test always starts with reading the Diagnostic Trouble Codes, AKA Onboard Diagnostic Codes(OBD Codes). The Onboard Diagnostic system was introduced and made mandatory for all vehicles sold in the United States since 1996, setting up a trouble code manual of all self-diagnostic results. Thus, for all cars sold after 1996, you can use a standard OBD2 scanner/reader to read the diagnostic trouble codes(DTC) that indicate the possible problems and the parts that are malfunctioning.

To do the job, you first should get a Car Diagnostic Tool. The obd2 readers on the market mostly are of two types:

1. Hand-held OBD2 reader

Hand-held OBD2 reader


2. Bluetooth OBD2 Scanner coupled with a software/mobile APP

Bluetooth obd reader


For a step to step guide of reading DTC codes, feel free to refer to this guide about what is OBD, how to read codes, and common codes you may see.


How often do you need to perform a car diagnostic test?

Although the vehicle check is required to be done every two years(it may vary based on the State), you may encounter problems every now and then. It’s really worrying when the engine light comes on or you hear or feel something is “off”, and you may take your vehicle to the mechanists just when it feels extremely wrong.

However, you don’t have to wait for the engine light to come on before you do a car diagnostic test. It’s recommended that you check your car for at least once a year, which will uncover minor problems that can’t be seen or heard (and that won’t trigger the check engine light). If you feel that taking the vehicle to the mechanists may cost too much or take too long, you can do it by yourself with an OBD2 reader. With the Diagnostic Trouble Codes read by the car diagnostic tool, you can search for the meanings and try several simple fixes depending on the code itself.


How to clear the engine light/DTC?

You can use an OBD2 reader to clear the code and turn off the check engine light. However, if you don’t solve the problem, the engine light will come back after one or more drive cycles. Also, if there are important or severe problems, it would be dangerous to drive around with a faulty vehicle.

Sometimes when there are minor problems and you let it be, major problems may occur and eventually cost more. So we highly recommend that you diagnose further into the Diagnostic Trouble Codes when they appear, and solve the problems by yourself or take it to a mechanic.



Car diagnostic tests are really important for vehicle health, and it’s recommended that you do it regularly to prevent minor problems from becoming big issues. Although bringing your vehicle to a mechanic will be a hassle-free choice, you can do it at home to save a bunch of money if you don’t see or feel something badly “off”. 

We highly recommend an OBD2 Reader with its bundled APP, ZUS. A free giveaway campaign is now ongoing, you can check it HERE. Don’t miss it!

Also, feel free to leave your comments and start conversations in the comment section below. 




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