What are DTC Codes & How to Diagnose Your Vehicle with DTC Codes?

In a perfect working system, there is usually a malfunction indicator lamp in place to inform users of any errors hampering the system’s performance. In the case of vehicles, DTC codes play the role of this indicator to let users know of any malfunctions.

Before DTC codes, a mechanic or owner would have to open their car’s hood up and fiddle around to see what was wrong. Though ever since the mandate for OBD 2 ports on vehicles the process of diagnostics has gotten easier and more efficient.    

Still, these codes can be confusing to those that have never used them. Thankfully these codes are very easy to understand once you see what they stand for. Let us see how you can decipher the meaning of these codes with ease.

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What are DTC codes?

DTC stands for Diagnostic Trouble Code. DTC codes are numerical codes used to diagnose problems in a vehicle’s engine control system and help in efficient repairs. DTC codes are integral to diagnosing the reason for any sort of vehicle downtime. This is because a DTC code or diagnostic trouble code gives you the exact cause of your vehicle’s malfunction. If your Check engine light becomes illuminated, this could signal any of several emissions-related faults your system has detected, such as a problem with your fuel system or evaporative emissions controls.

Knowing the exact cause of a vehicle malfunction can greatly increase the speed of repairs. This is why the universal interface to look at these codes was mandated for vehicle manufacturers. 

Whenever your vehicle’s check engine light is on you need to look at the diagnostic trouble codes to know what exactly is wrong. Looking at your DTC codes is a fairly simple process though most people do not know about it as they have not bothered to do it themselves.

How to Look at DTC codes?

Every single modern vehicle has an On-board Diagnostic system or OBD system that lets you read all the DTC diagnostic trouble codes in an instant. Whenever you have your check engine light on, it means that your vehicle’s on-board diagnostics system detects a malfunction that needs to be resolved.

Thankfully all of these systems share the same way to look at DTC codes. You can read all your diagnostic trouble codes by simply using the OBD-II port on your vehicle. Though if you operate heavy machinery with diesel engines, you will have to use a J1939 port.

1. How to scan OBD-II DTC codes?

Usually, in most OBD-II-compliant cars, the OBD-II port is located under the dashboard, near the passenger compartment area and steering column. All you need now is an OBD scanner to connect to the OBD-II port on your vehicle. To access DTC codes through the OBD-II port, you’ll need your vehicle identification number handy. You can find a pretty decent OBD scanner for less than 50 USD on most shopping websites. These scanners come at a relatively inexpensive price and could serve you for years.

After you connect the OBD scanner to your vehicle’s OBD port, all that is left to do is to scan it and look at every single diagnostic trouble code. Your vehicle might have all sorts of problems from a faulty sensor circuit to a faulty mass air flow sensor. This is why you need to know what DTC codes mean.

2. How to scan J1939 DTC codes?

Unlike OBD-II ports, the J1939 protocol has a different diagnostic connector and usually has its data port near the engine control module (ECM). The connector is also different and thus requires a separate scanner. These scanners, however, will cost a pretty penny. 

How to scan J1939 DTC codes

On average these scanners are 3x more expensive when compared to their OBD-II counterparts. Since they are completely different protocols, they don’t share the same DTC meanings. This is why you need to understand what DTC codes mean on both systems.

How to Diagnose Your Vehicle with a DTC Code?

Some indicators other than the check engine light give you insights into your vehicle’s problems such as the engine coolant temperature light or the ABS light. Other issues, such as a faulty idle control system, can also be spotted without DTC codes.

Though engine fault codes can give you a quick look at what exactly is causing the malfunctioning indicator lamp to flare up. This is why automotive engineers use them all the time to resolve issues. You too can do the same when you learn what every individual DTC code means.

How to diagnose your vehicle with OBD-II DTC codes?

How to diagnose your vehicle with OBD-II DTC codes

Diagnosing a car with OBD-II codes is pretty simple and straightforward, as all you have to do is look at all five characters in the corresponding trouble code and see what they mean. Each and every character has a variable value that tells you what exactly the issue is. Let us start with the first character and move on to the meanings of the rest.

1. First Character

The first character is an indicator for the area in which the malfunction is occurring and is usually denoted by an alphabet that relates to a specific vehicle telematics system.

  • Character P: Issue with the Powertrain system, including, the engine, gearbox, fuel system, and any other associated hardware.
  • Character U: Issue with networking systems or the vehicle’s onboard computers  
  • Character B: Issue with the car’s body or interior areas.
  • Character C: Issue that affects the chassis and related systems such as suspension, brakes, or steering. 

2. Second Character

The second character lets you know if the code is generic or standard DTC and has a standard meaning or if the codes are manufacturer-specific DTC codes. Usually, with manufacturer codes, you are doomed to scour the internet to know the meaning of the code.

  • Number 0: States that the code follows the generic code standardized by the society of automotive engineers (SAE)
  • Number 1: States that the code is manufacturer-specific code
  • Numbers 2 & 3: these digits are rare but they mostly mean that the code is again manufacturer specific. 

3. Third Character

The third character further refines by specifying the area of the issue that is affecting your vehicle, by using any number from 1 to 8. 

  • Number 1: Issue with the air metering and fuel system
  • Number 2: Issue with the injection of fuel or air
  • Number 3: Issue with the engine ignition system
  • Number 4: Issue leading to improper emissions
  • Number 5: Issue with the vehicle speed control system or engine rpm
  • Number 6: Vehicle computer and output circuit malfunction
  • Numbers 7: Issue with the transmission system
  • Numbers 8: Much like number 7 it denotes an issue with the engine transmission system

4. Fourth and fifth characters

The fourth and fifth numbers from the vehicle’s onboard diagnostics system will tell you the specific fault index number to pinpoint the underlying issue. These numbers can range between 0 to 99.

How to diagnose your vehicle with J1939 DTC codes?

If you operate any heavy-duty vehicles then you will most likely be dealing with J1939 DTC codes. These DTC codes work in a totally different manner when compared to DTC codes from OBD-II-compliant vehicles. 

They mainly use abbreviations to categorize issues in heavy-duty vehicles. These abbreviations function similarly to the characters in an OBD-II system where every character represents a problem. These abbreviations are as follows.

1. Suspect Parameter Number (SPN)

The Suspect Parameter Number denotes the area that is most likely to be the cause of the issue. All areas that can show up as vehicle errors will have an SPN and will be shown in the DTC code.

2. Failure Mode Identifier (FMI)

The Failure Mode Identifier code gives you the precise error that is occurring within the suspect parameter number. These errors can pop up due to a wide range of factors such as a calibration error or abnormal update rate. 

3. Occurrence Counter (OC)

The Occurrence Counter works even when the vehicle is not operational, to count the number of times that the malfunction has occurred. This gives you an insight into the time of the error popping, helping you identify the problem faster.

4. SPN Conversion Method (CM)

The Conversion Method is crucial to understand the meaning of all the codes above as their meanings rely on the conversion method used to derive them. The Conversion method tells you the byte alignment of the code by either a 0 or 1. 

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Deleting a DTC code will not solve the underlying problem that caused the DTC code to appear. Because fault codes only pop up with an abnormality on your car’s OBD system, it is important to pay heed to the code and see if it can cause a critical issue down the line.

While these codes are mostly universal and shared by the majority of car manufacturers, there are still some codes that are specific to the vehicle manufacturer. Such codes can be spotted by looking at the second character in the DTC code. If the second character is a zero, then it denotes a universal code.

There are plenty of Bluetooth-enabled OBD-II scanners that will wirelessly send all the DTC codes from your car to your Android/iOS phone. Such wireless OBD-II scanners can be a bit more expensive when compared to wired devices but they make up for it with their practicality.


The mechanical systems in a fleet vehicle are often neglected and overused to the point where they simply break down and cause long downtimes. This is why the check engine light pops up. It is so that you look at the DTC codes and figure out what is wrong with the vehicle before a critical issue ruins your day. 

DTC codes are truly a blessing for mechanics as, without properly understanding DTC codes, one is blind when fixing any modern vehicle. While regular vehicle maintenance is good to prevent breakdowns, you would have to ensure that your staff is equipped with the right fleet maintenance tips.

It is also of equal importance to be on the lookout for ways to improve the efficiency of your operations with software such as Upper. With Upper, an entire fleet’s routes can be made to perfection and dispatched within minutes. Try our 7 days free trial to see the abilities of Upper for yourself.

Author Bio
Rakesh Patel
Rakesh Patel

Rakesh Patel, author of two defining books on reverse geotagging, is a trusted authority in routing and logistics. His innovative solutions at Upper Route Planner have simplified logistics for businesses across the board. A thought leader in the field, Rakesh's insights are shaping the future of modern-day logistics, making him your go-to expert for all things route optimization. Read more.