UNDERSTANDING DTC CODES
What that Check Engine Light is Really Telling You 

Basic Guide to DTC Codes

There’s nothing quite like the sinking feeling of seeing your check engine light flick on—especially when you don’t know why it’s lit. Should you keep driving? Pull over? Call for roadside assistance? For drivers and fleet managers alike, this moment can trigger more questions than answers.

That’s where understanding DTC codes (Diagnostic Trouble Codes) becomes essential. With the right knowledge—and ideally, the right telematics tools—you can decode the issue quickly, take action, and reduce downtime or costly repairs.

Additionally, not all vehicle health issues (DTC codes) trigger engine lights.  Therefore, knowing the DTC codes present helps you take action, in time.

In this post, we’ll break down what DTCs are, how they work, and how to read them.

What Is a DTC Code?

DTC stands for Diagnostic Trouble Code—a standardized code used to identify specific issues within a vehicle. When your car or truck detects a fault, it logs the issue and triggers the Malfunction Indicator Lamp (MIL)—aka, the check engine light.

But while the light warns you something is wrong, it doesn’t tell you what. That’s the job of the DTC. These codes are stored in the vehicle’s onboard diagnostics (OBD) system and can be accessed using a diagnostic scanner.

Where Do DTCs Come From?

Every time a fault is detected, your vehicle’s OBD system not only activates the warning light but also generates a DTC that corresponds to the problem. These codes can then be read using a tool connected to your car’s diagnostic port.

There are two main diagnostic protocols used today:

• OBD-II – Standard for most light-duty vehicles
• J1939 – Common in heavy-duty vehicles and industrial equipment

To streamline diagnostics, the Society of Automotive Engineers (SAE) developed a set of universal DTCs for OBD-II-compliant vehicles. However, manufacturers often include their own custom codes for vehicle-specific systems—these are typically more complex and may require manufacturer-specific tools or expertise to interpret.

How to Read an OBD-II DTC Code

Each DTC is a five-character alphanumeric code. Let’s break it down:

Character 1: System Category

The first letter indicates which system the problem comes from:

• P – Powertrain (engine, transmission, emissions)
• C – Chassis (brakes, suspension, steering)
• B – Body (interior systems like airbags, lighting, AC)
• U – Network (communication between computers/modules)

Character 2: Code Type

This number tells you whether the code is universal or manufacturer-specific:

• 0 – Generic (standard across all manufacturers)
• 1 – Manufacturer-specific
• 2 / 3 – Less common, usually also manufacturer-specific

Character 3: Subsystem

This number identifies the affected subsystem:

• 1 – Fuel/air metering
• 2 – Injector circuit
• 3 – Ignition system
• 4 – Emissions controls
• 5 – Idle and speed control
• 6 – Output circuits
• 7 / 8 – Transmission

Characters 4 & 5: Fault Identifier

These two digits (00–99) pinpoint the specific issue, such as a faulty sensor, misfire, or circuit malfunction.

What to Do If You Get a DTC Code

If you or your drivers see a check engine light:

1. Use an OBD-II scanner to retrieve the DTC code.
2. Decode the code using a telematics system or by referencing the vehicle’s manual.
3. Determine severity—some issues are minor (like a loose gas cap), others could signal a major malfunction.
4. Take appropriate action—from scheduling maintenance to pulling the vehicle from the road.

For fleet operators, a telematics system that automatically reads and reports DTCs in real time can prevent small issues from becoming big ones.

Turn DTCs Into Data You Can Use

Understanding DTC codes isn’t just for mechanics anymore. With modern telematics, fleet managers and drivers alike can gain real-time visibility into vehicle health—making it easier to schedule maintenance, reduce breakdowns, and keep vehicles road-ready. 

Ready to take the mystery out of your check engine light? Frotcom’s fleet management platform includes automatic DTC diagnostics, service interval management and reporting.

Need help choosing the right telematics solution for DTC monitoring?
We’re here to help—contact us today for expert advice or to request a free demo.