Introduction
The OBD (onboard diagnostic) monitors the performance of different components and sends the codes to ECU. However, if the codes are out of the range, ECU is unable to read them and saves these codes in the form of DTC (diagnostic trouble codes). The DTC can thus be read using a hand code scanner or advanced code readers.
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These trouble codes are saved in alphabets and numbers that indicate the problem’s nature and source. Similarly, if ECU receives an unreadable code, it illuminates a warning light on the vehicle’s dashboard that indicates trouble in a particular part.
It is to be noted that you can quickly resolve some issues indicated by DTC by refilling certain fluids, while some problems can be complex and may require professional services. So, it is essential to know your engine fault codes and check them.
How does a DTC work?
As discussed that DTC is produced by the OBD when a vehicle’s component is malfunctioning. The OBD diagnoses the fault and illuminates the particular warning light, such as the check engine light on the vehicle’s dashboard. The mechanic or driver can connect an external scanner with the OBD connector to further analyze the DTC (4). This scanner can display the DTC, and some advanced scanners can also explore the performance of the faulty component.
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To diagnose a DTC, it is essential to detect the OBD system of the vehicle. DTC of OBD1 can change depending upon the manufacturer codes. But the cars that have integrated an OBDII have standard DTC specified by SAE (Society of Automotive Engineers).
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Onboard Diagnostics and DTC
Currently, two types of onboard diagnostics are used, and DTC can be analyzed according to these two OBDS (1):
- J1939: This OBD is usually integrated into heavy-duty diesel-operated vehicles
- OBD2: This OBD is integrated into light and medium-duty vehicles
So, if you own a simple hatchback, sedan, MPV, or SUV, your car has an OBD2 system, and DTC can be read according to the OBD2 specified codes. But if you have a heavy-duty truck, there is a J1939 system, and DTC codes are generated according to this system.
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It is also to be noted that the vehicles and light-duty trucks sold in the US are bound to be OBDII equipped. It is recommended that the drivers consult the vehicle’s instruction manual to analyze their onboard diagnostic compatibility.
How to Analyze an OBDII DTC?
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An engine fault code comprises five characters.
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The first character is an alphabet illustrating the source/control system of the issue (2). The second character is the number 0/1. A third character is also a number that indicates the malfunctioning subsystem. This letter can range from 1-8. The fourth and fifth characters are two-digit numbers indicating the exact issue. These two-digit numbers are also known as specific fault indexes.
i. First Character
As discussed above, the DTC code’s first character indicates the control system is facing an issue. There are four alphabets used to indicate the particular control system (3).
- The letter ‘P’ indicates the issue lies in the powertrain system. The powertrain includes an engine, transmission, and other associated accessories that drive the vehicle.
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So, if the DTC code starts with the letter’ P,’ the problem is within the main driving system, such as the engine, transmission, fuel system, etc.
The warning light will flash on the vehicle’s dashboard, illustrating the malfunctioning component.
- The letter ‘C’ indicates that the issue lies within the chassis or the components outside the passengers. The chassis includes features such as brakes, steering, suspension, etc.
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So, if the DTC code begins with ‘C,’ the problem is within the vehicle’s steering, suspension, or braking system.
The warning light will again indicate the specific issue on the vehicle’s dashboard.
- The letter ‘B’ indicates that the problem lies within the vehicle’s body or passenger component area. The body or passenger area includes airbags, seatbelts, and electrical components.
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So, if the DTC code begins with ‘B,’ there is an issue with the airbags, seat belts, respective sensors, or the electrical components within the passenger component.
Some warning lights indicate a particular problem in the passenger component. Usually, the seatbelt light or the airbag light flashes, indicating an issue within the passenger component.
- The letter ‘U’ indicates that the problem lies within the vehicle’s onboard computer or the related systems. This can include batteries, network buses, computers, etc.
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So, if the DTC code begins with ‘U,’ there is an issue with circuits, batteries, network buses, etc.
ii. Second Character
The second character depicts whether the DTC code is standardized or manufacturer specific (3).
If the second character of the DTC code is the number ‘0’, then it means that the DTC code is standardized.
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A standardized code implies that the DTC code is a generic code specified by SAE. These codes are followed by every car that has an integrated OBDII system.
If the second character of the DTC code is number ‘1’, then it means that the DTC code is unstandardized.
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An unstandardized code is specified by the manufacturer and differs for every model. These DTC codes are less common.
In rare cases, a DTC code can also begin with the number ‘2’ or ‘3’, which means that the code is manufacturer specific. But there can be a few exceptions.
iii. Third Character
A third character is also a number. This number in the DTC code indicates the particular subsystem that is malfunctioning. These numbers range from ‘1’ to ‘8’ (3).
- If the third character of the DTC code is number ‘1’, the problem lies within the fuel or air metering system.
- If the third character of the DTC code is number ‘2’, the problem lies within the air metering or fuel injection system.
- If the third character of the DTC is number ‘3’, it means that the problem lies within your vehicle’s ignition system.
- If the third character of the DTC is number ‘4’, it means that the problem lies within your vehicle’s emission system.
- If the third character is number ‘5’, your vehicle’s speed system or idling control functions are malfunctioning.
- If the third character is number ‘6’, the output computer systems are malfunctioning.
- If the third character is number ‘7’ or ‘8’, it illustrates any transmission-related issues.
iv. Fourth and Fifth Characters
The fourth and fifth characters are also numbers and are read together to figure out the specific fault index. These last characters thus specify the particular issue within the vehicle. The last three characters were more related to figuring out the malfunctioning part and subparts and the standardization (1). And the mechanic/driver should interpret the previous two numbers to specify the particular issue within the vehicle’s parts/subparts (3).
An Example of OBDII DTC Code and its Interpretation
The following table illustrates an example of the P0128 code interpretation (4):
| Number of Character | Specific Character | What does it Signify |
|---|
| First | P | The problem lies within the powertrain part of the vehicle. This means that the engine, transmission, or fuel system can malfunctioning. |
| Second | 0 | This means that the code is standardized, so the SAE manual can be used further to interpret the code instead of the instruction manual. |
| Third | 1 | This number illustrates that the problem lies within the vehicle’s air metering and fuel system. |
| Fourth and Fifth | 28 | The specific fault index is 28 indicating a low coolant temperature. |
So, the code P0128 means that the engine’s coolant temperature is below the thermostat regulating temperature.
How to Analyze a J1939 DTC Code?
A J1939 DTC code is indicated by heavy machinery or commercial vehicles. It usually comprises four field arrays (1).
- Suspect Parameter Number: These suspect parameter numbers illustrate the specific errors. There are different SPNs for different errors.
- Failure Mode Identifier: FMI illustrates the type and nature of the error that occurred in the heavy vehicle. This error can be a sensor short circuit, high engine temperature, calibration error, etc.
- Occurrence Counter: The occurrence counter stores a value indicating the number of times that particular error has occurred in the vehicle. It keeps the value of each SPN even when the error is alleviated.
- SPN Conversion Method: This array defines the byte alignment within the DTC. The value can be either 0 or 1, illustrating different types of CM.
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It is to be noted that a simple driver cannot comprehend the J1939 DTC codes. So, the heavy vehicle should always be taken to a professional service center for problem identification and resolution.
How can you read a DTC Code?
An engine fault code can be read using a multitude of tools. Some of these tools are listed below:
i. A Simple Hand Code Scanner
An OBDII DTC code can easily be comprehended using a simple hand-code scanner. This hand code scanner can be connected to the OBDII module using a USB connector. A simple hand code scanner will only display the code that is then to be comprehended utilizing the vehicle’s instruction manual or the online platforms.
ii. Advanced Code Scanners
Advanced code scanners are usually expensive and available with technicians. An advanced code scanner will not only display the code but will also decode it. Some advanced code scanners can also monitor the performance of the malfunctioned components. Through graphical monitoring, you can also monitor the real-time performance of an engine, transmission, etc.
iii. PC-Based and Mobile-Based Applications
Finally, you can also analyze the DTC codes using mobile-based or PC-based applications. The significant benefit of using mobile-based or PC-based applications is that they provide larger storage space. So, you can store the information on the mobile and PC to further analyze the performance. Further, the PC applications can also analyze the data in detail by graphing, plotting, and other techniques.
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It is to be noted that these devices can only decode the OBDII DTC codes. The mechanics have specific computer-based applications to solve a complex J1939 DTC code.
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Conclusion/Key Takeaway Points
So, DTC codes or engine fault codes can be of different types. If you have a light/medium duty vehicle of model 1996 or later, then the DTC code will be a standardized OBDII DTC code. These codes comprise five characters illustrating the type of code and the specific problem in the car. However, if you have a heavy-duty truck or vehicle, the engine fault code will be a J1939 code comprising four complex field arrays illustrating different information. Further, an OBDII DTC code can be decoded using a simple hand scanner, advanced hand scanner, or various mobile/PC-based applications. But the J1939 code is challenging to comprehend and requires professional technician services.
FAQs
Usually, the vehicles manufactured and sold in the US after 1996 are bound to integrate an OBDII system. Every DTC is standardized and indicates the same problem mentioned in the manual. However, if you want to know about the code, see the DTC’s second character. If it is ‘0’, the code is standardized, or vice versa.
Yes, if a warning sign is flashing on the vehicle’s dashboard, it indicates a DTC. It means that the OBD has sent a signal to ECU that is beyond its range and some vehicle component is malfunctioning. It would help if you always decoded the DTC to figure out the exact issue faced by the vehicle.
Yes, if the DTC is an OBDII DTC, you can quickly check it at home by connecting a hand-code sensor with the OBDII connector. But you cannot resolve some issues at home and might require professional services. Similarly, if DTC is a J1939 DTC, it can be complex to decode a DTC at home.
