OBD Trouble Codes Explained & How to Read Them


Aug 12th, 2021

OBD Trouble Codes Explained & How to Read Them

Onboard diagnostics in cars have come a very long way since the days of only having warning lights on the instrument panel. These days, OBD can provide you with specific trouble codes to quickly guide you to the root of the problem. 

How do you read an OBD trouble code?

OBD trouble codes are known as diagnostic trouble codes (DTC), which consist of five characters. The DTC will pinpoint the exact problem to its specific location, that is, the car system where the problem comes from. You can read these faults using an OBD scanner or OBD reader plugged into the OBD port below the dashboard.

In the following sections, you will discover everything you need to know about OBD codes, how to read them, and what they mean.

Let’s get to it.

Recap: OBD and Diagnostic Trouble Codes

We’ve covered the topic of onboard diagnostics (OBD) in much more detail in our previous article here. So, if you're unfamiliar with the subject, that article would make an excellent primer.

Still, let’s do a quick recap on what OBD is all about.

Onboard diagnostics or OBD is kind of like the warning lights on your instrument panel but on a whole other level. Instead of just flashing something like the Check Engine light coming on when there’s a problem, OBD uses plenty of sensors and the onboard computer to tell you about what’s going on under the hood.

That information includes errors and faults, emissions data, and more. All this information is available to you through the OBD port under your dashboard, which you can retrieve using an OBD scanner or reader.

Diagnostic Trouble Codes (DTC)

Suppose you plug your OBD scanner into the port and your car has a fault (or two). The onboard diagnostics will explicitly communicate that problem to you using a series of numbers and alphabets known as diagnostic trouble codes or DTC.

A DTC consists of five digits and typically looks like this 

P 0 3 02

The first three digits are individual, while the last two act as a pair. Each of those digits is important as they communicate a different aspect of the fault in question.

Using the same example above (P 0 3 02), here’s what each digit communicates:

  • The first digit: The trouble code subsystem. This digit refers to the different code families or code groups (which will be explored in detail in the next section).
    • The first digit consists of alphabets, specifically P, B, C, or U. These are explained in the next section.
  • The second digit: The type of code. Some codes are standardised, while others are specific to the vehicle's manufacturer.
    • So, this digit will be 0 (generic codes shared across car brands and models) or 1 (specific to the car’s manufacturer).
  • The third digit: The affected subsystem, i.e. where the problem is, like if the problem is with the fuel system, exhaust, transmission, or wherever else it might be.
    • This digit ranges from 1-8.
  • The fourth and fifth digits: The exact problem. For example, if there's an engine misfire, the code will let you know which cylinder is responsible for it.
    • This digit ranges from 01-99, which is why it’s a double-digit number, unlike the rest.

The code will tell you plenty about what might be wrong with your car when viewed as a whole. That allows for much quicker troubleshooting to find the correct solution and get the repairs done.

What Are the Different Families or Groups of Codes?

As you might imagine, OBD and the diagnostic trouble codes come with their fair share of pros and cons. Firstly, they’re amazing because of how specific they can be when highlighting a fault or problem. However, that also means it becomes a language that you must learn to interpret by referring to code lists.

Thankfully, the codes are very well organised and are grouped into a few different code families or code groups, making that process much more straightforward.

For example, the first digit of the code will help you narrow down the fault significantly. As mentioned above, the first digit communicates the Trouble Code Subsystem, also known as the code group or code family.

The four groups of codes are:

  • B: Body, i.e. there’s a fault with the car’s body
  • C: Chassis, i.e. the problem is related to the car’s chassis
  • P: Powertrain, e.g. you have an engine misfire or something similar
  • U: Undefined or Network Communication, e.g. your car’s entertainment system isn’t communicating with the car’s computer.

How to Read OBD Diagnostic Trouble Codes

As you can see, OBD diagnostic trouble codes are specific, but they’re not easy to read at a glance. So, learning how to read OBD codes takes a little bit of effort.

To read OBD diagnostic trouble codes, you will need two things:

  1. An auto code reader - This is known as a car code reader or OBD reader or scanner. The different names generally refer to the same device: one that you plug into your car’s OBD port under the dashboard to retrieve all the codes described above. To learn about auto code readers in greater detail, you can check out our previous article about them. 
  2. A code reference or database - By this, what we mean is that you need to have access to a reference document or database that tells you what the codes mean. Your car owner's manual should be the first place you check.

If you can't find what you need there, you could always check online. There are plenty of free and paid databases that offer generic codes and codes specific to your car brand and model.

Of course, never forget that you could also contact your car manufacturer's helpline for more information.

Limitations of Diagnostic Trouble Codes

Lastly, you must be aware that there are limitations to diagnostic trouble codes. First, yes, the OBD system is impressive overall. Plus, it’s still leaps and bounds ahead of those warning lights on your instrument panel that barely give you any helpful information.

Still, the codes that you get from the OBD aren’t always accurate. For example:

  • The ‘problem’ might not be a problem at all. It could just be that the wiring or sensor related to that part of your car is faulty and is essentially giving you a false positive.
  • The computer and sensors might be wrongly identifying the problem. Sure, there might indeed be a problem with your transmission (for example), but the OBD could be mistakenly showing you the wrong error code.

So, always remember that the onboard diagnostics and the trouble codes are just indicators. They are precise and handy indicators, but they can be imperfect and with limitations.

When you think you have the flu, the only person who can diagnose you accurately is a doctor. In the very same way, your OBD scanner will tell you what issues your car might have, but your mechanic is the only one who can confirm what that root issue might be.

To learn more about onboard diagnostics and trouble codes, check out the blog at Carpart.com.au.

By Ray Hasbollah