How to Use an Auto Scan Tool

Your Check Engine light is shining like a beacon of bad news. What's going on? We'll tell you how to figure it out on your own, on the cheap.

You're tooling down the road; it's a beautiful, sunny summer day, and to top it off, your car is running absolutely perfectly.

Perfectly, except for that Check Engine light shining like a beacon of bad news. What's going on? The car still runs well and idles fine. Maybe it's an electrical snafu. It's sure to turn itself off at some point, right? Which it does. And then it comes back, sort of at random. Now it's time to pass the state emissions inspection to renew your plates, and your car ain't gonna pass with that light on.

Back to Basics

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 Tuneups are kind of outdated these days because this computer system self-corrects the fuel mixture and ignition at every crankshaft revolution.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.

Checking Codes

To get started, plug the scan tool into the OBD II connector under the dash. Turn the key on, but don't start the engine. The tool will ask for a number of things such as the VIN, the make and model of the vehicle and the engine type. Follow the onscreen instructions.

You'll get an option to check for trouble codes, as well as a couple of other menu choices. Some higher-end scan tools give you a text explanation of the code onscreen. Others give you a CD-ROM or just a paper pamphlet listing what the codes mean. Your best bet is to search the Web for a trouble-code list on a site that focuses on your particular vehicle--there will be information that might be far more helpful than just the code. When you read these codes, write them down before resetting the MIL indicator.

Inspection and maintenance (I/M) readiness will likely be another menu option. If you've turned off the MIL, your car needs to be driven until the PCM is sure that the fault has been corrected. How long? Every manufacturer has its own ideas, but a few days of normal driving should fulfill what are called I/M readiness tests. Why do you care? Because if you go to the DMV or an independent station for an inspection, the technician will plug in his own scan tool, and if your car hasn't passed I/M readiness, there'll be no sticker for you. I/M readi­ness is to keep people from sneaking a poorly running car through the emissions test by clearing codes just before pulling up to the DMV.

So far, an inexpensive code reader (less than N15,000) tells you all of this, and even lets you clear the codes and turn off the MIL light. Don't confuse a simple code reader--which is sometimes labeled a scan tool--with a genuine scan tool. A code reader, however, might be enough to catch most simple problems.
Through a Scanner Darkly

Another menu option on your scan tool is to view data--here's the real functionality that can let you burrow down to the problem. You can page through the actual numbers the PCM is reading from the sensors. That allows you to check things like water and air temperature, oxygen-sensor function, engine load and rpm and a hundred other potential trouble areas. And you can do it in real time. There's also a freeze-frame function to trap that data when something hiccups while out on the road. You know, that misfire that goes away only when the mechanic is test-driving the car? With data like the ignition timing and injector pulse width available at the instant of the hiccup, your mechanic--or you--can probably figure out what happened.

Pro-grade scan tools have graphic functions that let a technician watch several parameters onscreen while the engine is running. Consumer-grade machines generally don't. But some higher-end ones, in the several-hundred-dollar range, have an interface for your laptop or even your desktop. No, you don't have to move your computer to the front lawn or garage--the scan tool has enough memory and battery life to let you plug it in indoors with the car cable disconnected.

There are also interfaces and software that let you hook up your computer directly to the OBD II connector via USB, allow you to look at the data on the computer screen and even make a printout.

PC interfaces allow you to replicate a scan tool with your laptop or desktop. The big graphic display is a plus, but to us, the laptop is a little too fragile for everyday shop use.

More sophisticated scan tools such as this give you more data, a bigger screen, alternate languages and the ability to interface with a PC for enhanced capability. These tools can be upgraded over the Web with the latest firmware.  You can also check your search for codes from Carpartsnigeria webpage from your phones ... Carpartsnigeria.com/obd_code
Your scan tool shows P0131, a general oxygen sensor code. No problem, you say, I'll just pop in a new sensor, which should clear up the problem, as well as the P0301 misfire code obviously caused by the bad sensor.