Guide to Using Car Code Readers

The benefits and limitations of code readers

Mechanic diagnosing a car

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A car code reader is one of the simplest car diagnostic tools you’ll find. They are designed to interface with a car’s computer and report trouble codes that can trigger check engine lights and other problems.

How Does a Car Code Reader Work?

OBD-I and OBD-II code reader systems work in essentially the same way, in that they monitor a variety of sensor inputs and outputs. If the system determines that anything is out of spec, it sets a “trouble code” that can be used in diagnostic procedures. Each code corresponds to a specific fault. There are also different types of codes (i.e. hard, soft) that represent both ongoing and intermittent problems.

OBD-II car code readers are typically inexpensive, but some parts stores and shops will read codes for free.

When a trouble code is set, a special indicator on the dashboard will light up. This is the malfunction indicator lamp (MIL), which is also referred to as a check engine light. It essentially means you can hook up a car code reader to see what the problem is. Of course, some codes do not trigger the check engine light.

Every OBD system has some type of connector that can be used to retrieve codes. In OBD-I systems, it is sometimes possible to use this connector to check codes without a car code reader. For instance, it’s possible to bridge GM’s ALDL connector and then examine the blinking check engine light to determine which codes have been set. In a similar fashion, codes can be read from OBD-I Chrysler vehicles by turning the ignition key on and off in a specific pattern.

In other OBD-I systems and all OBD-II systems, trouble codes are read by plugging a car code reader into the OBD connector. This allows the code reader to interface with the car’s computer, pull the codes, and sometimes perform a few other basic functions.

How to Use a Car Code Reader

In order to use a car code reader, it must be plugged into an OBD system. Each OBD-I system has its own connector, which can be located in a number of different places. These connectors are often found under the hood in the vicinity of the fuse box, but they can also be found under the dash.

In vehicles that were built after 1996, the OBD-II connector is typically located under the dash near the steering column. In rarer cases, it may be located behind a panel in the dash, or even behind the ashtray or another compartment.

car code reader obd2 connector
OBD-II connector plug. The port, which you will usually find under your dashboard, usually has the same shape, but some are rectangular.  Jeremy Laukkonen

Here are the basic steps for using a car code reader:

  1. Locate the OBD port.

  2. Insert the code reader's OBD connector into the OBD port.

  3. Turn the code reader on, if your unit doesn't power on automatically.

  4. Turn the vehicle's ignition switch to the accessory position.

  5. Follow the on-screen prompts on the code reader.

The specific procedure may differ from one code reader to the next. A lot of these devices are very simple, designed to turn on as soon as they receive power from the OBD port. Others have specific procedures you need to follow, in which case it's important to refer to the instruction manual.

What Can a Car Code Reader Do?

After the OBD socket has been located and connected, the car code reader will interface with the car’s computer. Simple code readers are able to draw power through an OBD-II connection, which means plugging the reader in may power it up as well.

At that point, you will typically be able to:

  • Read and clear codes.
  • View basic parameter IDs.
  • Check and possibly reset readiness monitors.

The specific options vary from one car code reader to the next, but at the bare minimum, you should be able to read and clear codes. Of course, it’s a good idea to avoid clearing the codes until you have written them down, at which point you can look them up on a trouble code chart.

Using a code reader to clear trouble codes also resets readiness monitors. Most emissions and smog testing facilities look at the readiness monitors and will not pass your car if they have been reset. If you clear your codes after completing a repair, it's important to take your vehicle on a test drive with a mix of city and highway driving, so that the readiness monitors have a chance to run.

Car Code Reader Limitations

Although car code readers are great at providing you with a jumping-off point for your diagnostic procedure, a single trouble code can have any number of different causes. That's why code readers are less useful than scan tools.

When a professional diagnostic technician uses a code reader, they often have prior experience with that type of code, giving them an idea of which components to test. Many professionals also have much more expensive and complicated scan tools, which come with huge knowledge bases and diagnostic instructions.

If you do not have access to such a tool, you can review basic trouble code and troubleshooting information online. For instance, if your car has an oxygen sensor trouble code, you would want to search for oxygen sensor testing procedures for your vehicle's make and model.

ELM327 vs. Car Code Readers

ELM327 scan tools represent an affordable alternative to basic car code readers. These devices use ELM327 technology to interface with your vehicle’s OBD-II system, but they don’t have any built-in software or display.

Instead, ELM327 scan tools are designed to provide an interface between a tablet, smartphone, or laptop and your car’s computer. Some mobile apps allow you to use an ELM327 scan tool and your phone as a bare-bones code reader, and more advanced software often provides you with a more powerful interface and diagnostic information.