How to Use a Car Diagnostic Tool

What to buy and what to check

In the past, car diagnostic tools were prohibitively expensive. Before 1996, an independent technician could expect to pay thousands of dollars for a tool that was compatible with only a single vehicle make. Even after the introduction of onboard diagnostics II (OBD-II), professional scan tools continued to cost thousands of dollars.

Today, you can buy a simple code reader for less than the cost of a movie ticket, and the right accessory can turn your phone into a scan tool. Since you can find most of the information you need to interpret trouble codes online, a check engine light no longer calls for an immediate trip to a mechanic.

Before you buy a car diagnostic tool, realize that they aren't some kind of magic panacea. When you plug in a check engine light code reader or a professional scan tool, it doesn't automatically tell you how to fix the problem. In most cases, it won't tell you what the problem is. It will provide a trouble code or several codes that provide a jumping-off point in the diagnostic process.

Person using a diagnostic tool on a car with open hood

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What Is a Check Engine Light?

When the check engine light turns on, your car is trying to communicate in the only way that it can. At a basic level, the check engine light indicates that a sensor (somewhere in the engine, exhaust, or transmission) provided unexpected data to the computer. That could indicate a problem with the system the sensor monitors, a bad sensor, or a wiring issue.

In some cases, a check engine light might turn on and eventually turn off with no outside intervention. That doesn't mean the problem has gone away or that there was no problem in the first place. Information about the problem is usually available via a code reader even after the light turns off.

How to Get a Car Diagnostic Tool

There was a time when code readers and scanners were only available from specialty tool companies, so they were difficult for the average vehicle owner to obtain. That has changed in recent years, and you can buy inexpensive code readers and scan tools from retail tool and parts stores, online retailers, and other locations.

If you aren't interested in purchasing a car diagnostic tool, you might be able to rent or borrow one. Some parts stores lend code readers for free with the understanding that you'll likely buy parts from them if you are able to figure out the problem.

Some tool stores and tool rental businesses can provide you with a higher-end diagnostic tool for less than it would cost to purchase one. So if you're looking for something beyond a basic code reader but don't want to spend the money, that may be an option.

The Difference Between OBD-I and OBD-II

Before buying, borrowing, or renting a car diagnostic tool, you need to understand the difference between OBD-I and OBD-II. Vehicles manufactured after the advent of computerized controls but before 1996 are grouped in the OBD-I category. These systems don't have a lot in common between different makes, so it's essential to find a scan tool designed for your vehicle's make, model, and year.

Vehicles produced after 1996 use OBD-II, which is a standardized system that simplifies the process. These vehicles use a common diagnostic connector and a set of universal trouble codes. Manufacturers can choose to go above and beyond the basics, resulting in manufacturer-specific codes. Still, the rule of thumb is that you can use any OBD-II code reader on any vehicle produced after 1996.

Finding Where to Plug a Diagnostic Tool

After you have your hands on a check engine light code reader or a scan tool, the first step in using it is to locate the diagnostic connector. Older vehicles equipped with OBD-I systems located these connectors in all sorts of places, including under the dashboard, in the engine compartment, and on or near a fuse block.

OBD-I diagnostic connectors come in a variety of shapes and sizes. If you look at the plug on your scan tool, you should get a good idea of what to look for in terms of the size and shape of the diagnostic connector.

If your vehicle is equipped with OBD-II, the connector is typically found under the dashboard to the left of the steering column. The position might vary from one model to another, and it can be difficult to find. In some cases, you may find that a panel or plug covers the diagnostic connector.

The connector is either rectangular or shaped like an isosceles trapezoid. It has sixteen pins that are configured in two rows of eight.

In rare cases, the OBD-II connector might be located in the center console behind the ashtray or in another difficult-to-find location. The specific position is usually recorded in the owner's manual.

Using a Check Engine Light Code Reader

With the ignition turned off, gently insert the code reader plug into the diagnostic connector. If it doesn't slide in easily, confirm that the plug isn't upside down and that you have correctly identified the OBD-II connector.

With the diagnostic connector plugged in securely, turn on the ignition. This provides power to the code reader. Depending on the device, it may prompt you for information at that time. You may need to enter the VIN, the type of engine, or other information.

At that point, the code reader is ready to do its work. The basic devices provide stored codes, while advanced scan tools give you the option to read the trouble codes or view other data.

Interpreting Check Engine Light Codes

If you have a basic code reader, write down the trouble codes and do some research. For example, if you find a code P0401, a quick internet search reveals that it indicates a fault in one of the oxygen sensor heater circuits. That doesn't tell you exactly what's wrong, but it's a good place to start.

Advanced scan tools might tell you exactly what the code means. In some cases, it can provide you with a troubleshooting procedure.

Next Steps

Whether you have a basic code reader or a fancy scan tool, the next step is to determine why the trouble code was set in the first place. The simplest way to do this is to look up potential causes and rule each one out in turn. If you can find an actual troubleshooting procedure, that's even better.

In the earlier example of a P0401 trouble code, further investigation reveals an oxygen sensor heater circuit malfunction in bank one sensor two. A malfunctioning heater element could cause this, or it could be a problem with the wiring.

In this case, a basic troubleshooting procedure is to check the resistance of the heater element, either confirm or rule out a problem there and then check the wiring. If the heater element is shorted or shows a reading out of the expected range, replacing the oxygen sensor would probably fix the problem. If not, then the diagnostic would continue.

Finishing the Job

In addition to reading codes, most check engine light code readers can also perform a few other important functions. One such function is the ability to clear all stored trouble codes, which you should do after you attempt a repair. That way, if the same code comes back later, you know that the problem wasn't fixed.

Some code readers and all scan tools can access live data from a variety of sensors while the engine is running. In the event of a more complicated diagnostic or to verify that a repair has fixed the problem, look at this data to see the information from a specific sensor in real time.

Most code readers can show the status of individual readiness monitors. These monitors reset automatically when you clear the codes or when the battery is disconnected. This is why you can't disconnect the battery or clear the codes before having your vehicle's emissions tested. If you do need to go through emissions, it's a good idea to verify the status of the readiness monitors first.

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