How to Use an Auto Scan Tool ?

In 1996 the EPA mandated that the computer interface for all vehicles sold in the United States meet a common standard. This meant that aftermarket repair shops didn't have to buy a half-dozen different $2000-plus scan tools, one for each brand of car they wanted to work on.
The standard is called OBD II (On-Board Diagnostics, version II). The fuel injection, ignition system and automatic transmission on most modern fuel-injected cars and trucks are run by one or more computers. These monitor sensors that collect data from the engine and other systems all over the car. The computers send commands to the fuel injectors and ignition coils to fire the cylinders. They use the data to fine-tune the combustion process with the correct amount of gasoline and the correct ignition timing to provide efficient, clean combustion for good power, economy and low pollution. There are dozens of sensors, measuring such things as throttle opening, engine rpm, air and coolant temperature, crankshaft and camshaft position, and road speed.

It's a sophisticated, high-speed system with a lot of self-checks to keep your car running as intended for a long, long time. Tuneups are kind of outdated these days because this computer system self-corrects the fuel mixture and ignition at every crankshaft revolution. This is much better than it was decades ago, when you'd take the family bus down to the garage for that annual tuneup.

Sounds great, right? And it is--until there's a malfunction. Often, your first indication of anything awry in the engine room is that Check Engine light on the dash. The OBD II standard tags this light as the MIL (malfunction indicator lamp). When that light is on, it means that the PCM (powertrain control module) is seeing data from some sensor that, well, doesn't compute--it's far enough out of bounds that the PCM doesn't know what to do. So the PCM guesses what should be a good number to substitute for the sensor's erratic output and presses on, getting you home from work. It also illuminates the MIL so you'll know something just isn't right. In spite of this, your car should run surprisingly well, although you may notice a slight loss of power and increased fuel consumption.

There's more. The PCM also stores a diagnostic "trouble code" so a service technician has some clue where to look for the problem. Scan tools, used to check these codes, were long the province of professional technicians because they were expensive and dedicated to one manufacturer's vehicles--a scan tool for a VW, for example, wouldn't work on a Chrysler. Scan tools are now inexpensive enough for car owners to buy. Prices vary from 20 bucks or so for a mindless code reader that only shows trouble codes, to maybe $400 for full-featured, upgradable machines with computer interfaces and multiple languages. Or, you may just be able to borrow a scanner from the auto parts store for an hour or two.