Instructables
It’s state inspection time and you’re ready to leave bright and early for the inspection station with the family’s ’98 minivan. You’re starting the day in a less than jovial mood because your daughter said “the light came on” last night on the way home. She said she threw a few dollars of gas into the tank to make it home. Turns out your annoyance is misplaced. Sure enough, the light’s on all right, but it’s not the fuel warning, it’s the Check Engine warning. You stop for gas anyway, and then you pull into the inspection lane.

Well, full tank or not, the minivan does not pass muster.

Now what?  This Instructable will help you use scan tools to diagnose your car's problems.  You can do the basics yourself before taking the car in to a mechanic.

This project was originally published in the August 2001 issue of Popular Mechanics.  You can find more great projects at Popular Mechanics DIY Central.
 
 
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Step 1: Use an OBD II

Picture of Use an OBD II
This is a late-model vehicle with what’s known as OBD II, the second-generation on-board diagnostic system that replaced OBD I starting in 1994. It’s industry-wide and federally mandated. One of the problems with OBD II for the do-it-yourselfer is that you can’t get trouble codes by counting the blinks
on the Check Engine light like you can on earlier computer-controlled vehicles. You could take your minivan into a high-tech shop, where the minimum charge for diagnosis and inspection could run you somewhere into three figures. Or you
can learn about OBD II yourself, but you will need a piece of diagnostic equipment that you probably don’t have—an OBD II scan tool. 

If the Check Engine light came on, it should be no surprise that the vehicle failed an emissions test. With OBD II, that light comes on only if there’s a failure that significantly affects emissions. That makes the scan tool even more important since it will reveal a lot of the problems that do not cause the warning light to come on.

With many problems, the light stays on after the repair is made and the code remains in the computer memory for a certain number of ignition on/off cycles. Your daughter didn’t tighten the gas cap correctly, causing a code for the evaporative emissions system to set. Eventually the light will go out and the code will self-erase, perhaps after the next time you start and stop the car. You also can use the scan tool to erase it immediately.

With many other problems, however, the only way to turn off the light and erase the code is with the scan tool. Just a warning: If you erase trouble codes with a scan tool or disconnect the battery for any reason, you also erase the computer’s continuous monitoring system. So if you take your car in for a state inspection before enough normal driving, the computer might not have completed all its tests, and your car will fail inspection for that reason.

The OBD II scan tester not only will enable you to find answers to the simpler problems, but will tell you into what areas the seemingly more complicated ones fall. Then you will have a better understanding of what the technician is (or should be) looking for.
PS1181 year ago
FYI, you don't even need a dedicated tool anymore. Now you can get a little bluetooth module for like $10-$20 (USD) and it beams live speedometer/fuel economy/error codes straight to an app (Torque is a popular one) on your android!

Naturally there is a similar apple set-up as well.