How to Use a Car Diagnostic Tool

What to buy – and what to check

how to use a car diagnostic tool
Your phone can act as an inexpensive car diagnostic tool with the right accessory, but it won't fix your check engine light for you. PeopleImages / E+ / Getty
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In the past, car diagnostic tools were prohibitively expensive. Prior to 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 even turn your phone into a scan tool.

Since most of the information you’ll need to interpret trouble codes can be found online, a check engine light no longer has to call for an immediate trip to your mechanic.

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

What is a Check Engine Light?

When your check engine light turns on, your car is trying to communicate in the only way that it can. At the most basic level, the check engine light indicates that some sensor, somewhere in your engine, exhaust, or transmission, has provided unexpected data to the computer.

That could indicate a problem with the system the sensor is responsible for monitoring, a bad sensor, or even a wiring issue.

In some cases, a check engine light may turn on and then eventually turn itself off with no outside intervention. That doesn’t mean the problem has gone away, or that there was no problem in the first place.

In fact, information about the problem is usually still available via code reader even after the light turns itself 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 somewhat 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 many other locations.

If you aren’t interested in purchasing a car diagnostic tool, you may even be able to rent or borrow one. Some parts stores readily lend out code readers for free, with the understanding that you’ll probably buy some 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 tools for much less than it would cost to purchase one. So if you’re looking for something beyond a basic code reader, but you don’t want to spend the money, that may be an option.

The Difference Between OBD-I and OBD-II

Before you buy, borrow, or rent a car diagnostic tool, it’s also important to understand the difference between OBD-I and OBD-II.

Vehicles that were produced after the advent of computerized controls, but prior to 1996, are all lumped together in the OBD-I category. These systems don’t have a lot of common between different makes, so it’s vital to find a scan tool that was specifically designed for the make, model, and year of your vehicle.

Vehicles produced after 1996 use OBD-II, which is a standardized system that simplifies the process a whole lot. These vehicles all 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, but 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

Once 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 also come in a variety of shapes and sizes. If you look at the plug on your scan tool, you should be able to 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, then the connector will usually be found under the dashboard to the left of the steering column. The position can vary from one model to another, and they can also be buried pretty deep. In some cases, you may find that the diagnostic connector is even covered by a panel or plug.

The connector will be either rectangular or shaped like an isosceles trapezoid. It will also have sixteen pins that are configured in two rows of eight.

In rare cases, your OBD-II connector may even be located in the center console, behind the ashtray, or in other difficult to find locations. The specific position will usually be recorded in the owner’s manual if you have difficulty finding it.

Using a Check Engine Light Code Reader

With the ignition key turned off or removed, you can gently insert your code reader plug into the diagnostic connector. If it doesn’t slide in easily, then make sure the plug isn’t upside down and that you have correctly identified the OBD-II connector.

With the diagnostic connector plugged in securely, you can insert your ignition key and turn it to the on position. This will provide power to the code reader. Depending on the specific device, it may prompt you for some information at that time. You may need to enter the VIN, the type of engine, or other information.

At that point, the code reader will be ready to do its work. The most basic device will simply provide you with any stored codes, while other scan tools will 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, you will have to write down the trouble codes and do some research. For instance, if you find a code P0401, a quick Internet search will reveal 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.

Some scan tools are more advanced. If you have access to one of these, the tool may be able to tell you exactly what the code means. In some cases, it will even 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 your 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.

Taking the earlier example of a P0401 trouble code, further investigation would reveal that it indicates an oxygen sensor heater circuit malfunction in bank one sensor two.

This could be caused by a malfunctioning heater element, or it could be a problem with the wiring.

In this case, a basic troubleshooting procedure would be 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 that is out of the expected range, then replacing the oxygen sensor would probably fix the problem. If not, then the diagnostic would continue.

Finishing the Job

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

Some code readers, and all scan tools, can also 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 actually fixed the problem, you can look at this data to see the information from a specific sensor in real time.

Most code readers are also capable of showing the status of individual readiness monitors. These monitors are automatically reset when you clear the codes or when the battery is disconnected. This is why you can’t just disconnect the battery or clear the codes before having your emissions tested. So if you do need to go through emissions, it’s a good idea to verify the status of the readiness monitors first.