Until recently, a tool like the Innova 3100 OBD II code reader was not typically found in the home mechanic’s toolbox. For working on modern cars, however, it can have real utility.

Many people like to tinker with their cars, and are quite capable of doing so. But one major difference between older and current cars is the use of a computer control system to manage and monitor the operation of the engine. Such a system consists of an on-board computer and associated sensors, switches and actuators.

In addition, and this is where a code reader comes in, vehicles sold in North America since 1988 have been required to use an on-board diagnostic (OBD) program capable of identifying and memorizing faults in the computer control system and the components it manages.

The first generation of on-board diagnostics, called OBD-I, monitored engine-operating data, including trouble codes, for up to 30% of a vehicle’s components. Beginning in 1994, OBD-I was phased out and gradually replaced with the second-generation OBD-II system.

By 1996, all vehicles used the OBD-II system. It monitors up to 85% of components from fuel injection to ignition to catalytic converter. It’s one of the ways a technician identifies the reason your pesky “check engine” light comes on.

Having access to the data stored within the computer control system would enable you to diagnose codes before taking the car for service, or having it emissions tested (although this isn’t the method for testing actual exhaust gasses).

But as I say, if you don’t have access to testing equipment, you can’t use these systems to diagnose faults, and until recently, the required equipment has been too expensive for consumers to buy.

The hand-held Innova 3100 OBD-II code-reader is a comparatively inexpensive way to download a car’s trouble codes. It works with all cars using the OBD-II monitoring system. Here’s how you use it:

The code-reader comes with a cable (it looks like a printer cable for your computer) that connects to a plug located in your car, typically located beside and below the steering wheel. The device is controlled with only four buttons, and displays information on a 2.5 cm x 4.0 cm liquid crystal screen. The four buttons are Power, Link, Scroll and Erase.

Once connected, you press the Power button to turn on the Innova 3100. Then you turn the car’s ignition on without starting the motor, and subsequently press the Link button on the code-reader. After a few seconds, the codes stored in the OBD memory, are downloaded to the reader, where you can scroll through them using the Scroll button.

There’s a manual supplied with the Innova 3100 that decodes readings from Ford, Chrysler, GM, Honda and Toyota vehicles, although as mentioned earlier, it downloads codes from all OBD-II vehicles.

If you don’t have a vehicle from one of the five manufacturers in the manual, you’ll need to order a service manual for your car from the parts counter at your dealer. It will decode the readings you download.

I asked the technician at my local garage to try out the Innova 3100, and he found it useful. The machine he currently uses requires additional steps for downloading codes from individual manufacturers. His professional system set him back a few thousand dollars about five years ago, although prices have come down for all systems since then.

If you drive a car that uses the earlier OBD-1 system, Innova also sells model 3123 for GM, 3145 for Ford, 3163 for Chrysler and 3173 for Import vehicles.

If in doubt about what model to buy, OBD-I or OBD-II, check your vehicle’s Vehicle Emissions Control Information label located under the hood or by the radiator of most vehicles.

The Innova 3100 OBD-II code-reader retails for $249.99. OBD-1 code-readers retail from $31.99 to $44.99. They’re available at Canadian Tire, Wal-Mart and UAP/NAPA stores. Occasionally, they’re on sale, so check around for the best price.