Diagnostic trouble codes

Diagnostic trouble codes for OBD II explained. The standardized diagnostic codes provide a detailed description of the faults detected by the powertrain control module. This article will provide information about what each digit stands for in the standard 5-digit code. This information will aid in check engine light diagnosis.

If you are looking for a list of definitions for check engine codes. I do provide that from this next link. Note that this is not a complete list, but actually a list of the most common fault codes that I have seen in the last few years. You can review the page that contains the most common diagnostic trouble codes.

When you're browsing through the list of any OBD II trouble codes you'll find that there is gaps in the numbering. That will allow codes to be added in the future.

Also the list of codes includes some equipment not included on all engines and this may also provide gaps in the numbering system. The importance of standardized codes is so that every car make and model will use the same check engine code number for a particular fault.

Five digit diagnostic trouble codes

If the first letter is a P, this will stand for powertrain computer. If the first letter were a B, this would stand for the body computer.

The second character is a number that indicates whether the trouble code that follows is an OBD II generic code or a manufacturer specific code that is also known as an enhanced diagnostic trouble code.

If the second digit is a zero it is a generic code. If the second digit were a one then this would be an enhanced or manufacturer specific Check engine light code.

The third character indicates the subsystem of the area that the fault lies in. The fourth and fifth characters identify the specific faults detected in that subsystem.

Let's use an example of the common P0306. The first letter P indicates a powertrain problem. The zero indicates that it is a generic code.

The number three indicates that the failure is in the misfire monitoring system. The last two digits indicate that it is the number six cylinder that has been detected as misfiring.

Sensitive diagnostic trouble codes

OBD II codes are stored according to priority. The DTC with a higher priority will set with the first indication of a failure.

Diagnostic trouble codes that are not a high priority may only set after two or three faults have been detected. These may be shown as pending codes on your automotive scan tool.

Some diagnostic car scanners will also display how many times a pending code has failed. Compared to the amount of active drive cycles in the memory.

This could mean that a body code may have shown a fault one time out of the last 75 key cycles.

In the case of a P0306, if the number six cylinder misfired one single time the malfunction indicator lamp would be turned on immediately.

This is why I recommend clearing the code and seeing if it returns. Codes that turn on the check engine light on the first failure are usually related to powertrain emission systems.



Give this OBD II diagnostic trouble code page a Then move on for more information.



I have a lot more information about OBD II diagnostic trouble codes, including some of the common problems that will illuminate your check engine light



The best way to approach diagnosis of a trouble code is to get a service manual for your vehicle. The most comprehensive information is available instantly online. I put together a video of how to use online car repair manuals.



Search through both of my auto repair websites and the Internet.

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