DTC Codes: How They Work + How To Identify Them

Is your check engine light illuminated?

There are many triggers for this — a faulty O2 sensor, a solenoid malfunction, an engine misfire, a loose fuel cap, an airbag problem, and so on.

So what is the problem?
The best way to narrow it down is to take a look at the DTC code.

Not sure what that is?
In this article, we’ll explain DTC codes in detail — how they work, how to extract them, interpret them, and answer some FAQs.

Let’s start!

What Are DTC Codes?

DTC stands for Diagnostic Trouble Codes.

These codes are used by your vehicle’s onboard diagnostics (or OBD system) to identify a problem or a malfunction your vehicle experiences.

There are tons of DTC codes, each representing specific vehicle problems.
That’s why, if you have a basic understanding of DTC codes, it can help you better figure out your car troubles and what to do about them.

Let’s find out how DTC codes work.

How Do DTC Codes Work?

Whenever your car experiences a problem or detects a fault, a DTC code is generated by your vehicle’s OBD system.

Once detected, the OBD system will alert you by displaying the DTC code via visible warnings like an illuminated check engine light (Malfunction Indicator Lamp).

A DTC code lets external devices, such as an onboard diagnostics scanner (code reader), interact with your car’s OBD system.

Initially, there were several versions of onboard diagnostics interfaces that largely varied depending on the vehicle manufacturer.

But today, there are two main standards used to read DTC codes:

OBD-II: For light and medium-duty vehicles
J1939: For heavy-duty vehicles and equipment

With the implementation of OBD-II, the Society of Automotive Engineers (SAE) created a standard DTC list full of common codes for every vehicle manufacturer. These codes help cars stick to emission regulations and save the environment.

Note: OBD-II may be used the most, but DTC codes specific to certain manufacturers do exist. A vehicle manufacturer may create their own DTC codes to add to the universal code list if certain vehicle models need it.

But the question is, what do these OBD codes mean?
And how do you read them?

Let’s find out.

How To Interpret OBD-II Codes?

Don’t let the OBD codes and their thousand variants intimidate you. All you need to do is understand the DTC code structure.

They’re pretty standard, with five characters in each DTC code.
Let’s understand the structure of OBD codes one character at a time:

1. The First Character: Alphabet

An OBD-II diagnostic trouble code starts with an alphabet denoting the car part with a fault.

P (powertrain): Covers transmission, engine, fuel system, and other associated accessories
C (chassis): Includes mechanical systems and functions outside the passenger compartment like steering, suspension, and braking
B (body): Car parts that are mostly found in the passenger compartment region
U (network and vehicle integration): Functions controlled by the OBD system

2. The Second Character: Number

The second character of an OBD-II DTC is a number that tells you if the diagnostic trouble code is standardized. It’s either 1 or 0:

0 denotes a standardized SAE international code. It’s known as a generic code or global code (sometimes).
1 denotes a manufacturer specific code. It’s also known as an enhanced code.

3. The Third Character: Number

For a powertrain code, this number helps you identify if you have a malfunction in any vehicle subsystems. There are eight numbers to denote a subsystem malfunction:

0 indicates fuel and air metering as well as auxiliary emission control
1 denotes fuel and air metering
2 represents fuel and air metering (injector circuit)
3 denotes the ignition system or a misfire
4 indicates auxiliary emission control
5 denotes vehicle speed control, idle control system, and auxiliary inputs
6 indicates computer and output circuit
7 represents transmission

Note: For other codes denoting other car parts, refer to the definitions given by your vehicle manufacturer. If there’s any manufacturer specific code, you’ll find it on the website of your vehicle manufacturer as reference material for your vehicle model.

4. The Fourth And Fifth Characters: Number

The fourth character of an OBD-II diagnostic trouble code is a digit indicating the exact problem or malfunction your car is experiencing.

It can be a number between 0 and 99.

Here’s an example of a complete DTC fault code:

The P0573 code is a generic code about the powertrain and is defined as “Cruise Control/Brake Switch A Circuit High.”

How To Extract DTC Fault Codes?

To extract and read DTC fault codes, you need a diagnostic connector and a DTC scanner. The DTC code reader or scanner shows codes in one-line descriptions or includes code definitions.

Diagnostic connectors aren’t universal, so if your vehicle diagnostic connector were manufactured before 1996, it’d need specific adapters.

Newer car models typically have the same connectors.
Here’s how to extract DTC codes from your car:

Plug the scanner or code reader into your car’s 16-pin OBD-II diagnostic connector. You can find it on the left of the steering column, under the dashboard.

Then turn on the ignition to let the scan tool communicate with your vehicle’s electronic control unit (ECU).

With the scan tool plugged in, access the tool’s menu, select the option for “read codes.” Your scanner will automatically detect your vehicle model and year. If it doesn’t, it probably isn’t set to do so automatically. In such a case, you’ll have to enter the specifics manually.

The scan tool will then show any DTC fault codes that may exist. You can transfer the diagnostic trouble codes to a smartphone or computer using Bluetooth or Wi-Fi if you have the latest scanner.

The DTC codes you extract will offer some insightful information about your possible vehicle problems. But you need to remember that these codes are more like a guide to help you spot underlying issues to diagnose and fix the malfunction.

Now that you know what DTC codes are and how they work, let’s answer some FAQs.

5 FAQs on DTC Codes

Here are some common questions related to DTC codes and their answers:

1. Is My Vehicle OBD-II Or J1939 Compliant?

If your car was manufactured and sold in the US after January 1, 1996, it’s OBD-II compliant. The same goes for light trucks.

Most vehicles with a diesel engine and equipment like heavy trucks and buses are J1939 compliant.

The easiest way to confirm if your vehicle is OBD-II or J1939 compliant is to check your vehicle manual or reach out to your dealership to verify.

2. What Are Some Common DTC Codes?

Your car may have a malfunction or problem for various reasons — depending on its condition, the weather you drive in, etc. However, there are some DTC codes that a driver may often encounter.

These include:

P0442: Small system leak within the car’s evaporative emission control system
P0300: Random or multiple cylinders misfire detected
P0215: Engine shutoff solenoid malfunction
P0650: Malfunction Indicator Lamp (check engine light) control circuit malfunction
P1108: Dual alternator battery lamp circuit malfunction
P1794: Battery voltage circuit malfunction
B1927: Faulty passenger side airbag
B1203: Fuel sender circuit short to battery
P0352: Ignition coil B primary or secondary circuit malfunction
P0353: Ignition coil C primary or secondary circuit malfunction
P0130: O2 sensor (oxygen sensor) circuit malfunction (Bank 1, Sensor 1)
P0141: O2 sensor (oxygen sensor) heater circuit malfunction (Bank 1, Sensor 2)
P0654: Engine RPM (revolution/rotations per minute) output circuit malfunction
P0120: Throttle pedal position sensor or switch A circuit malfunction
P0656: Fuel level output circuit malfunction

3. Are All DTC Codes Critical?

Many diagnostic trouble codes can illuminate your check engine light, but not every trouble code is critical. How critical a code is, is driven by what system the malfunction affects.

You can divide DTC codes into two categories:

1. Critical Codes

Critical DTC codes need urgent attention because they can cause severe damages in no time to your car.

For example, a DTC code indicating high engine temp or low coolant levels is critical because it implies that an engine failure is impending.

2. Non-Critical Codes

A non-critical DTC fault code isn’t urgent, but it still needs the right diagnosis.

For example, any emission fault can be a non-critical error code that turns on the Malfunction Indicator Lamp (MIL) or check engine light.

4. How Can I Clear DTC Codes?

The easiest way of clearing codes is to fix the problem, and it’ll automatically get erased.

Before OBD-II was introduced, many vehicles let car owners disconnect the battery cable or the computer’s power source to clear the OBD codes.

How?
Disconnecting the battery or power source means no voltage, which would clear the computer’s temporary memory, and the check engine light would turn off too.

However, the latest OBD system stores the DTC codes in a memory that doesn’t erase, even if the battery or computer’s power source is disconnected.

Every fault code stays in the memory until you or your mechanic clear them with a scan tool.

5. What Are Permanent Diagnostic Trouble Codes?

Permanent Diagnostic Trouble Codes (PDTC) are much like diagnostic trouble codes, except they can’t be erased or cleared using an onboard diagnostics scan tool.

Fixing the underlying problem of the vehicle that prompted the permanent diagnostic code, and its corresponding DTC, is the only way to clear it.

Final Thoughts

An illuminated check engine light can cause panic.
However, if you know the source of the problem, you can easily get help from your mechanic.

Alternatively, you can just reach out to an automobile and maintenance service provider like AutoNation Mobile Service.

AutoNation Mobile Service is a mobile auto repair solution with hassle-free online bookings as well as affordable repairs and replacements. They can help you decipher the DTC codes and fix the problems.

Reach out to them, and AutoNation Mobile Service’s ASE-certified mechanics will come to your doorstep and fix any car problem no matter the DTC code!