While your scan tool is connected, next check for any other stored or pending codes that may be related. For example, other sensors supply needed information to the ECM to correctly calculate the amount of fuel to add. If these sensors are reporting falsely, fuel trim will be affected. Codes related to engine performance can also impact fuel trim.

Once you have gathered this basic information, resist the urge to clear the codes. There is still information in there we may yet need. Clear the codes only when you are ready to verify your repair. (See "Think Like the PCM," Motor Age, January 2007).

So far we've seen what data is available, what it means, and how it's used by the ECM. We've also seen how you should gather it for interpretation. Next month, we'll see how you put all this information together using your off-board diagnostic tool — otherwise known as your brain — to understand what the ECM thinks is happening and why.

Pete Meier is an ASE CMAT, a member of iATN, and a full-time tech with CarMax in Tampa, FL. He started doing oil changes and minor repairs more than 30 years ago and brings a variety of experience to bear. His current job involves all manufacturers' lines, and, as Pete says, "provides me constant opportunity to learn something new." Diagnosing electrical and driveability problems are his favorite challenges.