Guide to Using Car Code Readers

The benefits and limitations of code readers

Mechanic diagnosing a car

ferrantraite / Getty Images

A car code reader is one of the simplest car diagnostic tools you’ll find. They're designed to interface with a car’s computer and report trouble codes in a very no-frills sort of way.

How Does a Car Code Reader Work?

Computer controls started to show up on cars in the late 1970s and early 1980s, and these systems rapidly grew in complexity. Even very early computer controls included basic on-board diagnostic (OBD) functionality, and these early, OEM-specific systems are collectively referred to as OBD-I. In 1995, for the 1996 model year, automakers around the world started transitioning toward the universal OBD-II standard, which has been in use ever since.

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

Both OBD-I and OBD-II 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, and there are also different types of codes (i.e. hard, soft) that represent both ongoing and intermittent problems.

When a trouble code is set, a special indicator on the dashboard typically lights up. This is the malfunction indicator lamp (MIL), which is also referred to as a check engine light. It essentially just means that you can hook up a car code reader to see what the problem is. Of course, some codes won’t cause this light to turn on.

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.

Using a Car Code Reader

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

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 an ashtray or another compartment.

car code reader obd2 connector
This is what an OBD-II connector plug looks like. 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 you need to follow to use 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 can differ from one code reader to the next. A lot of these devices are very basic in nature, and they are designed to turn on automatically as soon as they receive power from the OBD port. Others have specific procedures that 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 hooked up to, the car code reader will interface with the car’s computer. Simple code readers are actually able to draw power through an OBD-II connection, which means that plugging the reader in may actually power it up and turn it on 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 another, 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 good 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 that allows them to know which components to test. Many professionals also have much more expensive, and complicated, scan tools that come with extensive knowledge bases and diagnostic procedures.

If you don’t have that sort of tool at your disposal, then you can check out 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 specific make and model of vehicle.

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, display, or anything else that a traditional code reader has.

Instead, ELM327 scan tools are designed to provide an interface between a tablet, smartphone, laptop, other devices, and your car’s computer. Basic free 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.