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What are Vehicle Diagnostics?

Diagnostic Trouble Codes are codes that the vehicles on-board diagnostics (OBD) system uses to notify you about an issue in various areas of your vehicle. Each code corresponds to a fault which has been detected in your vehicle and when the vehicle detects an issue, it will activate a corresponding trouble code.


A vehicle stores this code in its memory when it detects a component or system that is not operating within the manufacturers acceptable parameters. The code will help to identify a fix regarding the issue within the vehicle. Each trouble code consists of one letter and four digits, such as P1234.


Format of the OBD2 Trouble Codes


System or Category

The OBD2 Trouble Codes are categorised into four different systems.

  • Body (B-codes) category covers functions that are, generally, inside of the passenger compartment. These functions provide the driver with assistance, comfort, convenience, and safety.

  • Chassis (C-codes) category covers functions that are, generally, outside of the passenger compartment. These functions typically include mechanical systems such as brakes, steering and suspension.

  • Powertrain (P-codes) category covers functions that include engine, transmission and associated drivetrain accessories.

  • Network & Vehicle Integration (U-codes) category covers functions that are shared among computers and systems on the vehicle.


The first letter of the code will mark the system related to the trouble code.


Generic and manufacturer specific codes

The first digit of the code tells you if the code is a generic or manufacturer specific code.

Codes starting with 0 as the first digit are generic or global codes. It means that they are adopted by all cars that follow the OBD2 standard. These codes are common enough across most manufacturers so that a common code and fault message could be assigned.

Codes starting with 1 as the first digit are manufacturer specific or enhanced codes. It means that these codes are unique to a specific car make or model. These fault codes will not be used generally by the majority of manufacturers.

The first digit might be also 2 or 3. In this case the type depends on the system. B2xxx and C2xxx codes are manufacturer controlled while B3xxx and C3xxx codes are currently reserved. P2xxx codes are generic codes while P3xxx codes are manufacturer controlled. U2xxx codes are manufacturer controller as well as U3xxx codes.


Subsystem or functional area

Previously, the second digit defined the sub-system of the codes. However, the latest document defining the diagnostic trouble codes (J2012 revised in 2016-12) had some changes to this.

According to the document, as the DTC usage has increased with the introduction of new technology to vehicle systems, it was necessary to remove the grouping of DTCs into functional areas.


Fault description

The last two or three digits define the actual fault description. These numbers will inform the technician of the problem and each code is defined separately. Generally, people only become aware of the On-Board Diagnostics protocol when the Malfunction Indicator Light (MIL) lights up on their dashboard. The MIL is also known as the Check Engine Light and the purpose of this warning light is to indicate a problem has been detected with the vehicle car and to alert the driver of there being an issue which needs to be checked out.


What does the Malfunction Indicator Light mean?

The Malfunction Indicator Light can signal three different types of problems.

  • Occasional flashes indicate temporary or intermittent engine malfunctions. In this case it would make sense to be aware of possible future issues which can later evolve into more serious ones.

  • The most common case is when the indicator light stays on constantly. This would indicate a more serious problem that requires immediate action to be taken. However, occasionally the issue may not be that serious but may affect the emissions of the vehicle and negatively influence an MOT inspection, fuel and power efficiency.

  • The most serious type of signalling is when the MIL flashes all the time. It is a sign that your vehicle’s engine is misfiring. The issue is a major one and you should stop the engine immediately to prevent serious damage. For instance, it might cause the catalytic converter to overheat and even cause a fire.

How to get more information about the issue?

Some OBD-II issues are of lesser importance and do not have much impact on the vehicles operation. On the other hand, some of the issues could be very serious and need the appropriate measures to be taken. Unfortunately, there is no way to distinguish between them by just looking at the light that has popped up on the dashboard.

The only way to find out what the real issue is and how much of a problem it could be or will become is for the vehicle to undergo a diagnostic scan. Although the OBD2 system can turn the MIL off automatically should the issue disappear, it is more usual for either the light to return or just to stay on.

AutoMotive Services provide a full manufacturer diagnostic scan facility.

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