What is a diagnostic trouble code (DTC)?

HomeGlossary of fleet termsWhat is a diagnostic trouble code (DTC)?

Diagnostic trouble code (DTC) definition

When a car drives down the road, various sensors send data points to the central computer of the vehicle. If the data falls out of a healthy range or reports a misfire in the car’s operations, it could trigger a check engine light. This alerts the driver that something is wrong with the vehicle’s performance that needs to be addressed.

To understand why the check engine light turned on, a mechanic can look at the diagnostic trouble code (DTC) which explains the part of the vehicle that is malfunctioning. DTCs are part of OBD-II systems and allow for universal troubleshooting of motor vehicle issues. They are also different from a J1939 code, which addresses the electronic control unit (ECU) of a vehicle.

Both companies and drivers need to be aware of DTCs, how to check them and how to make necessary repairs so the check engine light turns off. This is a key part of keeping drivers safe and improving the performance of enterprise fleets.

How do DTCs work?

Several components of a vehicle need to work together for it to operate smoothly. If even one aspect of the operations is not working, the vehicle’s performance can start to decline. When a car is turned on, the internal computer constantly checks on various systems to make sure they are working correctly. When something is wrong, it sends an alert to the driver through the check engine light.

DTC issues can be large or small. If the check engine light turns on and stays on, the problem isn’t critical but should be addressed at the next destination. For example, if an oxygen sensor breaks, the car can keep running. However, if the check engine light continues to flash, the driver should pull over immediately. Critical issues with the engine or drivetrain could disable the vehicle in an unsafe environment.

When a check engine light flashes, a technician will need to read the code. Each number and letter in a DTC means something different and these codes can even vary based on the manufacturer. This means a fleet manager or mechanic needs a code reader to understand the problem.

For reference, the state of Delaware created a useful guide that explains common DTCs used by most vehicle models. Here is how most DTCs are structured:

  • It starts with a letter. If a code starts with a P, B, C or U then the problem lies with the powertrain, body, chassis or network. This tells you where the issue is.
  • The next number is either a 0 for a universal code across all cars or a 1 based on a manufacturer-specific issue.
  • The next number provides more details on the location of the problem. This could range from the ignition system to the transmission.
  • The final numbers specify exactly what went wrong and what needs to be addressed.

When reading DTC codes, know that the letters and numbers start generic and get more specific. This allows technicians to start troubleshooting on a high level and then zoom in on the problem.

How to read a DTC

Various maintenance codes require specific code readers. You need to make sure your service staff can read DTC, OBD and J1939 codes to understand the types of maintenance your vehicles need.

Fortunately, there are ways to streamline the code reading process to better understand the issues with your vehicles. An integrated telematics platform can display all relevant information available in a central location. Multiple team members can also have access to this information through their corporate devices.

When your team spends less time trying to figure out the issue, they can spend more time addressing it and working to get the vehicle back on the road.

The importance of reading DTCs?

Unless the check engine light is flashing, a vehicle can technically continue to drive even if a DTC needs to be addressed. Not every problem will completely disable a vehicle. However, it is in the best interest of companies to address check engine lights and read the DTCs when they are sent.

The main benefit of reading DTCs is driver safety. You want to feel confident that your staff is driving a road-ready vehicle every time they get behind the wheel. Checking DTCs can potentially save lives if your team can catch problems early on.

There are also financial benefits of reading DTCs. Cars don’t repair themselves and small issues can turn into major problems if they are left unchecked. Making repairs to your vehicles in a timely manner could potentially lower your long-term repair costs and extend the life of your fleet.

This is one of the benefits of adding fleet tracking software to your vehicles. Your in-house team can receive alerts when there is a DTC thrown by a vehicle. You and your staff can act immediately to send that vehicle to the service department and assign a new car or truck to that driver for the immediate future.

DTCs and fleet management

DTCs do not have to disrupt your operations. If you have a dedicated process in place, you can address these codes and return your vehicles to the road within a few days. Here are a few ways fleet managers can stay on top of DTCs.

  • Create a reporting system for drivers to highlight problems before a DTC code fires. If a vehicle is driving strangely, it is better to catch an issue ahead of time.
  • Develop a maintenance schedule to address potential issues before a DTC is thrown.
  • Set up alerts so in-house teams know about DTCs are soon as they fire.
  • Keep backup vehicles ready so team members can continue working while their main car or truck is getting repaired.
  • Take steps to improve your routes to reduce disruption if multiple vehicles need repairs at the same time.

Along with fleet management, GPS asset tracking software can also support these initiatives across any commercial vehicles and other mobile equipment requiring maintenance and compliance inspections. Fleet-tracking and GPS tools can help automate some of your processes so addressing DTCs isn’t a labor-intensive process.

Responding to DTCs can make your team feel safer behind the wheel while potentially reducing your long-term fleet maintenance costs. Evaluate your processes to address vehicle problems and see how fleet management tools and other technology can help your business run more smoothly.

Our solutions

View all
DOT Compliance

DOT Compliance

Simplify compliance for your driver’s legal obligations, help reduce the paperwork burden and encourage worker safety with Verizon Connect DOT compliance solutions.

GPS asset tracking software

GPS asset tracking software

Monitor your valuable equipment with Verizon Connect GPS asset tracking software. Track and manage your high-value assets with one powerful equipment tracking software solution.

Awards and recognition

2024 Silver Stevie Award for Company of the Year

Automotive & Transport Equipment

AutoXreport Innovator Award 2024

EV Suitability Tool

2023 Gold Globee Award for Artifical Intelligence Solutions

AI Dashcam

2023 Bronze Stevie Award for Company of the Year

Automotive & Transport Equipment

CompassIntel Award 2023

Artificial Intelligence: Enterprise Solution 2023

MFA 2022

Best Telematics Solution for Fleet Managers Italy 2022


Top Innovator 2022

IoT Evolution Awards

Product of the Year 2021

One Planet Awards

Gold 2020

Stevie Awards

Silver 2020

IT World Awards

Gold 2020

Golden Bridge Awards

Gold 2020


4.2 Star Rating

Green Industry Pros

Editor’s Choice 2019

Contractors Top 50

New Products 2019

Best in Biz

Silver 2019

IOT Innovator

Gold 2019

The Golden Bridge

Gold 2019

One Planet

Gold 2019