Your car can tell you more about what's wrong with it than you might realize—you just need the right communication tool. A handheld diagnostic code reader can help you determine the cause of some problems and even identify potential issues that don't yet exhibit obvious symptoms. Armed with this insight, you may be able to make a fix yourself, or you can use the information to ensure that a mechanic does a fair, cost-effective repair.

This information exchange is made possible by an onboard diagnostics system that monitors a vehicle's electronic control system and can alert you to related problems in the engine or transmission. When it detects a malfunction, the system automatically illuminates a “check engine” light in the instrument cluster and logs a diagnostic code in its computer that indicates the problem’s location.

Mechanics can use a code reader to access those diagnostic codes, which helps them to identify and fix the problem. Similar devices are also available for consumer use. Consumer Reports tested a group of those code readers, ranging in price from about $80 to $180. We evaluated each for its ability to read and detail problems correctly, and for ease of use. We found that all but one of the readers correctly detected all error codes generated in our testing, but some had more features that made them easier to use and more expensive.

Like most electronic devices, code readers inhabit a quickly changing world. New features and models have been added since our test, and one of the units has been discontinued. While we have not tested the new models, we thought a quick update might help.


GETTING A READ ON OBD-II

The On-Board Diagnostics system, or OBD, is a technology that developed from the rapid expansion of car computerization in the 1980s. As a vehicle's engine-management computer was asked to control more operations, including myriad powertrain, chassis, and safety items, there was a need for a common interface for mechanics to use. The current industry standard is called OBD-II, which has been used on all cars built since 1996.

Federal law mandates that a vehicle’s OBD-II engine-control system use a standard set of codes that are common to all cars. That allows you to use one reader to access the core diagnostic codes for any vehicle. Some automakers, though, will add additional codes that are specific to their vehicles, and the translation might not be readily available.

The OBD-II port is usually found under the dashboard near the steering wheel. See your owner's manual for the location. Using a code reader is a simple matter of finding the port and plugging in the reader. The ignition must be switched on for the device to work. If the reader detects a problem, a numerical code, or depending on the model, a brief description of the problem, is displayed on the device. By understanding the code, users can determine whether the problem requires an immediate trip to the mechanic or is relatively minor. Mechanically-savvy owners might even be able to make repairs.


WHAT WE TESTED

To evaluate the devices, we sought volunteers whose vehicles’ “check engine” light had been staying on during normal driving. Among the 14 vehicles we used, one had a light that had been on for two years. As a control, we used a 15th vehicle whose light was not on during normal use, but we unplugged a sensor in that vehicle to cause the light to come on. That enabled us to check the devices against a known cause.

We tested six code readers on each vehicle. Five of the readers—the Actron CP9125, Actron CP9135, AutoXray Code Scout 1500, Innova 3030, and Innova 3100—perform the same four functions:

We also tested the CarChip E/X, a combination code reader and vehicle-monitoring device. It can record specifics about the way a vehicle is being driven, such as straight-line acceleration and braking.


WHAT WE FOUND

While all five dedicated code readers worked well, some were easier to use than others. The Actron CP9125 ($80) and the Innova 3030 ($80) are basic, budget-priced units with limited features. Both are compact in size, which might be a selling point for some users. But pocket-sized also means small screens, fewer controls, and buttons that perform more than one function, which can be confusing and awkward. The Innova 3030 is better in that regard, with one button for scrolling through functions—something not as easily accomplished with the CP9125. Neither unit will translate codes, requiring a user to look them up on the code-reader manufacturer’s Web site, product manual, or CD-ROM. Both have short cords, which can make them harder to use depending on the location of your car’s OBD-II port.

Models with more features are available from both companies at a higher price. But one we tested, the Actron CP9135, is no longer being made. The new CP9175 has some new features, including the ability to take a “snapshot” of the vehicle’s operating condition at the time of the faulty code. That records things like engine speed and temperature to assist in diagnosis, but we have not tested its effectiveness. The discontinued CP9135 did well in our testing.

For its additional cost, the Innova 3100 ($130) doesn't offer much more than the 3030. It has a longer, detachable cord and a backup battery that allows a user to unplug the device without losing a displayed code. Since the unit doesn't provide problem descriptions, the battery is handy when you have to take the reader away from the car to look up the code.

The AutoXray Code Scout 1500 ($150) was the most comprehensive product we tested and has the most intuitive, easy-to-use controls. Like the Innova 3100, it has a long, detachable cord and a battery that enables a user to unplug the cord without losing the information on the display. The large screen is easy to read, and descriptions are provided with codes.

The CarChip E/X ($179) was the only device of the six that failed to read some of the codes generated during our testing. It can reset an engine light and perform some diagnostic functions, but it cannot perform I/M readiness testing or report the status of the engine light. The CarChip E/X does not have a screen to display error codes. That is a significant drawback because information must be downloaded to a computer to be read.

The CarChip E/X might be of interest to some buyers because it can record driving behavior, but we found it limited as a maintenance and repair tool.


CR'S TAKE

The code readers we tested are all useful aids to home mechanics and are easy enough to use that they can be beneficial even to less-skilled car owners. At the least, they can give you an idea of what is wrong with your car before you take it to a repair shop, which helps you to be better informed and can help you avoid being ripped off for unnecessary work. Letting a mechanic know what information you got from the reader can also help him find and repair a problem faster.

If you can fix the problem yourself, a code reader could also save you money. Something as simple as a loose fuel-filler cap, for example, could cause the “check engine” light to go on. But even if you tighten the cap, you might need to have the light reset. Some of the volunteers in our test reported being charged up to $100 to have that done. But any of the models we tested can reset the light. With prices for our devices beginning at less than $100, one fewer visit to the shop could pay for the reader. If you own multiple vehicles or can share a reader with family and friends, the investment is even more sound.

For a backyard mechanic who wants the most complete product, the AutoXray Code Scout 1500 is a good choice. If you don’t mind buying a product that is no longer being made, the Actron CP9135 might be available online. It offers most of the features of the AutoXray at a much lower price—especially now. But it pays to shop around before buying any code reader. Like most electronic devices, they vary widely in price.

A final reminder: A check-engine light can indicate a serious problem. Whether you use a code reader or take the vehicle to a service center for a professional diagnosis, it is recommended that the problem be identified.

See the related report: What to do if the “check engine” light goes on.