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When the check engine light (CEL) comes on, there is usually a diagnostic trouble code (DTC) that can be read from the OBD-II (onboard diagnostic) port on modern codes.

How do I go about using the code to figure out what might be wrong with my vehicle?

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    Seems like we're getting a lot of "how do I fix <insert DTC code here>?" type questions. I'm thinking it would be good to put together a DTC 101 answer that we can point people at to get started and to help them refine their questions so that we can give better answers. – dlu Dec 12 '16 at 1:29
  • Good question! We can probably close most obd questions as a dupe of this provided the answers cover all the ground necessary. – anonymous2 Dec 12 '16 at 15:16
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The computers in a car are constantly monitoring the operating conditions and sensors that are in the car. When a test fails or something goes out of a pre specified range a DTC is set.

The biggest misconception about a DTC is that it tells you what to replace, it does not. A DTC tells you what test failed or what parameter went out of range. This identifies the where something failed but not why. It drives me insane when parts stores read codes and give you parts to replace. Their job is to sell you parts not diagnose cars. For example P0401 EGR flow insufficient is a common code. The knee jerk reaction tends to be to replace the EGR valve. A more common failure is when the EGR passages plug up causing the insufficient flow. All the EGR valves in the world won't fix plugged up passages.

There are two basic routes for diagnosing codes. The first is the manufacturer service manual. The service manual contains diagnostic routines for every code that the vehicle can set. These diagnostic routines give step by step instructions on what to check and how. These manual can be found on line or bought. Service facilities have subscriptions to websites like Alldata, Mitchel and Identifix which are repositories for all manufacturer service manuals. Unfortunately the average person can't afford these websites. When diagnosing a code that you're not familiar with this is always the first stop. Following these diagnostic routines will find the exact problem 99% of the time. The problem is that 1% where something goes wrong that the engineers who wrote the diagnostic routines could not account for.

This brings us to the second way. This method involves raw experience. After you do this for a while, you find certain patterns in failures. This allows you to jump to a specific test without going through the entire diagnostic routine. This is also where that 1% falls. For example P0402 EGR flow excessive, or EGR flow during non EGR condition. This code is commonly caused by a plugged up catalytic converter. Older diagnostic routines did not include checking this as a procedure. Later as it was found out that this is a problem this step was added to newer manuals. (Once a manual is printed it is effectively set in stone, they won't reprint another one to update it.)

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For the DIY mechanic without access to sites like Alldata or the experience of a professional mechanic, I'd like to add a third option to @vini_i's answer, and that is a simple google search with your make, model, year, and the fault code. I have found that often times a specific make and model will have a common failure that results in a fault code. You can sometimes find a forum where other owners of the same vehicle have reported what they had to do to fix their vehicle, sometimes there is a pretty good consensus. You can also find advice for common failure reasons in general. e.g. the P0402 code mentioned in @vini_i's answer. Even this site has answers to several fault code questions (https://mechanics.stackexchange.com/search?q=error+code)

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