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Understand Diagnostic Trouble Codes. The Ultimate Guide

The check engine light itself does give the driver a rough idea that something is probably wrong, though it’s not specific. To find out what the issue is, you will need to check the car’s Diagnostic Trouble Codes.

Although there are thousands of different DTC codes, some of which are manufacturer or vehicle-specific, understanding them is a lot easier than it may seem at first. This short guide will help you find out all you need to know about Diagnostic Trouble Codes in your vehicles.

What Is A DTC Code?

DTC codes are universal codes that can help you easily identify vehicle-related issues. Diagnostic Trouble Codes or DTC does for short, are a useful feature that can be found in any vehicle with an OBD-2 port. Every single vehicle manufactured from 1996 onwards has one of those.

Diagnostic Trouble Codes, often simply referred to as “fault codes”, are made up of five digits. Once a vehicle fires a DTC code, the MIL (malfunction indicator lamp, a.k.a. the check engine light) will go off to notify the driver. However, the check engine light itself won’t tell you what exactly the problem is.

This is where DTC codes come in handy. Once you plug a device into your vehicle’s OBD-2 port (more information on that below), you’ll be able to check which Diagnostic Trouble Codes caused the check engine light to go off. You can then proceed with repairs, if necessary, or clear the codes and continue driving. Either way, it’s always better to know what the issue is before driving off.

How Can You Read DTC Codes?

You will need to connect to your vehicle’s OBD-2 port to read the Diagnostic Trouble Codes. In order to do that, you require a diagnostic tool or a scanner.

Just around a decade ago, you would have to go to a repair shop to get your check engine light diagnosed. As you can expect, that wasn’t exactly convenient nor cheap. Luckily, those days are long gone.

Movitronic Keybox One is an advanced device that plugs right into the OBD-2 port of your vehicle. Once Keybox One is installed in your vehicle, you’ll be able to read DTC codes as soon as they go off. It’s the ideal solution for both fleets as well as private car owners.

Pro tip: Some manufacturers allow drivers to read DTC codes without the need for any external devices. Owners of Dodge RAM trucks from the late 90s, for example, can simply switch the ignition on three times in a row. Older vehicles will blink the check engine light (count the number of blinks to identify the code), whilst newer ones will display the code in the digital odometer.

Are DTC Does Universal?

Yes! Every vehicle fitted with an OBD-2 port will display the same set of universal DTC codes, no matter the make or model.

Note that although DTC codes are indeed universal, not all trouble codes will apply to every single vehicle. P0442 is a code that will go off if a small leak in the vehicle’s evaporative system is detected. Naturally, this exact code will never pop up in an electric car.

What Are The Two Types Of Diagnostic Trouble Codes?

There are many different ways to categorize DTC codes. However, it is generally agreed upon that all Diagnostic Trouble Codes belong to one of two types:

  • Type A- severe issues that typically require urgent repairs
  • Type B- minor issues

You can also group Diagnostic Trouble Codes based on the first character of the code itself, as that refers to the part of the vehicle that needs attention and possibly repairs. Take a look at the paragraph below for more details.

How Can You De-Code Diagnostic Trouble Codes?

Believe it or not, decoding a DTC error is easier than it may seem. To keep things simple, we’ll go over it character-by-character. Keep in mind that a DTC code is 5 characters long.

Let’s use P0138 as an example.

The First Character — P, C, B, U

The first character of a DTC is always a letter.

  • P- Powertrain. Codes starting with a P refer to the vehicle’s motor, transmission, fuel system, and anything closely linked to said components.
  • C- Chassis. These codes refer to the vehicle’s mechanical systems that are usually outside of the cabin. Think driveline, suspension, and braking systems.
  • B- Body. Body-related issues and problems. Pretty self-explanatory.
  • U- Network. These errors are linked to the car’s computer and anything that’s closely related to it.

With that in mind, we can see that P0138 is an issue related to the car’s powertrain.

The Second Character — 0, 1, 2, 3

The second character in a DTC code is a number from 0 to 4. It will let you know how universal the error is.

  • 0- In this case, it’s a generic code that you’ll find in every other OBD-2 vehicle. Standardized by the SAE.
  • 1- The code is specific to either the make or model. If it went off in an Audi, you will not find the same code in a BMW.
  • 2 and 3- These codes are very rare, and their meaning depends on the first character in the DTC code. Most of them are manufacturer-specific, too.

Our code, P0138, is a generic powertrain-related issue.

The Third Character — 1 To 8

The third character in a Diagnostic Trouble Code is another digit. This one will help you identify what system is at fault. 

  • 1- fuel/air metering and auxiliary emissions
  • 2- fuel/air metering injection controls
  • 3- ignition or misfires
  • 4- emissions
  • 5- vehicle speed controls, idle control, auxiliary inputs
  • 6- computer and output circuit
  • 7 and 8 describe transmission failures

Looking at the information above, it is clear that P0138 is linked with an issue related to fuel/air metering and auxiliary emissions.

The Fourth & Fifth Character

The last two characters of a DTC code are digits that will help you narrow down the issue as much as possible. These two-digit codes are specific to your vehicle, so you’ll need to look up the index codes for your automobile to de-code them.

After checking the index code, we can see that P0138 indicates an issue with the O2 sensors- high voltage in bank 1 sensor 2, to be precise.

Can You Clear Diagnostic Trouble Codes?

Yes! However, you should never neglect any DTC codes that pop up in your vehicle. Be sure to repair the issue first, then clear the codes if necessary.

You can use Movitronic Keybox One to clear DTC codes in all vehicles connected to the device. Note that some manufacturers require the ignition to be switched off and on again in order to clear the codes.

Fleet Management Vs DTC Codes

Keeping track of DTC codes is a crucial element of proper fleet management. Back in the old days, fleet managers would have to rely on drivers to report any vehicle-related issues. The manager would then have to schedule the vehicle for repairs. Luckily, there are products that have made this process easier than ever before.

Movitronic Keybox One is a universal solution that plugs right into your vehicle’s OBD-2 port. Once the device is set up, you have access to a wide array of information, such as the car’s real-time location or any DTC codes that have fired. You can even unlock the car doors remotely using your phone!

Kenneth Pohl
Co-founder & CEO of Movitronic

Kenneth has more than 10 years of experience in sales, mobility and technology.

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