Diagnosing automotive problems used to be much more challenging. A mechanic had to use all of their skills to pinpoint mechanical or electrical issues. Nowadays, you can diagnose many problems electronically. With an OBD scanner and a little know-how, anybody can identify precisely what’s wrong with their vehicle.

But how exactly does a scanner work, and how do you use one? Here’s an overview of the basics.

Modern cars have an onboard diagnostic (OBD) computer connected to various sensors. When one of the sensors detects an issue with your vehicle’s systems, it notifies the OBD. The computer then activates your Check Engine light.

In and of itself, the Check Engine light isn’t helpful. It’s a catch-all light that lets you know there’s a problem – a defective sensor can even activate it. You don’t know whether the problem is your ignition, transmission, emissions, electronics, etc. But by using an OBD scanner, you can get to the root of the problem.

When your OBD computer detects a problem, it stores a five-character code corresponding to that issue. These codes, known as OBD or OBD-II codes, are the key to diagnosing your issue.

To read your OBD codes, you’ll first need to obtain an OBD scanner, a handheld device with a 16-pin plug that connects to your car. It also has a screen that will visually display any error codes.

Your OBD scanner will have a connection cable with a 16-pin trapezoidal plug at the end. This plug goes into your vehicle’s OBD port, also referred to as the diagnostic link connector (DLC). Different cars will have varied locations for the OBD port. You’ll often find it underneath the steering wheel, somewhere near the pedals. Less often, it’s located towards the center of the dashboard or even at the passenger side. If you’re having trouble finding it, look to your owner’s manual.

Insert your key in the ignition, but don’t start the engine. Instead, turn your key only to the point where the car’s power comes on. This will activate the OBD computer, so your scanner can communicate with it. Depending on your scanner, you’ll see a visual message letting you know the link is being established.

If nothing happens, double-check your connection to ensure that the scanner you’re plugged in all the way. If there’s still no activity from the computer, plug something into your car’s cigarette lighter and see if it’s getting power. The cigarette lighter and OBD computer are on the same circuit, so this will let you know if there’s something wrong with the electrical circuit. In that case, you’ll need to replace the corresponding fuse.

Some OBD scanners will automatically detect what type of vehicle you’re scanning. Others will require you to enter that information manually. Some scanners will simply have a menu for selecting the make and model. Alternatively, you may have to enter your vehicle identification number (VIN).

Now that you’re connected, you’re ready to read your codes. How exactly you do this will be different depending on the scanner. In most cases, you’ll find the codes in the menu under “Codes” or “Trouble Codes.”

With many scanners, you’ll be able to view active and pending OBD codes separately. An active code represents a current issue causing your Check Engine light to illuminate. Pending codes represent errors but have not caused the light to activate. If the same error happens again, the pending code will be upgraded to an active code and activate the light.

At this point, you’re ready to decipher any OBD codes that appear on your screen. Each code consists of five characters, and each character has its meaning.

The first character specifies what area of the car is having trouble. The most common one is P, which indicates a problem with your powertrain system. C indicates a problem with the chassis, such as your brakes. B covers any issue with the vehicle’s body, including safety devices like airbags. U, meanwhile, means that there’s a problem with the car’s communication system.

The second character lets you know whether the OBD code is generic or manufacturer-specific. Generic codes apply to issues that could happen on any car. For example, you’ll see the same code for failed anti-lock brakes on any vehicle. Manufacturer-specific codes apply to problems with parts that only exist on specific models.

The code is generic if the first two characters are B0, B3, C0, C3, P0, P2, P34-P39, U0, or U3. On the other hand, it's a manufacturer-specific code if the characters are B1, B2, C1, C2, P1, P30-P33, U1, or U2.

The third character will give you more precise information on where the issue is. The same third character can have different meanings depending on the first two characters. For powertrain issues, in particular, a 1 represents a problem with fuel or air monitoring, while a 2 represents problems with fuel or air injection. A 3 indicates an ignition problem, and a 4 means there’s trouble with the emissions system. A 5 indicates speed or idling control issues, and a 6 represents a computer problem. Numbers 7-9 are used for transmission problems.

The last two digits provide the exact nature of the problem. There are thousands of codes – far too many to list here. But there are plenty of online sources where you can look up OBD codes for free.

As you can see, using an OBD scanner is a simple, straightforward process. You’ll soon understand why your Check Engine light is illuminated by following these simple steps. After that, all that remains is to perform the necessary repairs.

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© 2022 J.D.Power. All rights reserved.

© 2019 J.D.Power. All rights reserved.