Q: I took your advice and purchased a diagnostic scanner. I’ve been playing with it and trying to make sense of some of the things on the data list. I remember you saying that the “fuel trims” were a useful thing to check. How do they work?

A: Fun stuff! Cars built since 1996 (1997 for trucks) are required to display the air-fuel correction factor, among other system operating data on an onboard diagnostic scan tool. This will typically be seen as a “short-term fuel trim” percentage and a “long-term fuel trim” percentage (some manufacturers list these as a decimal value). For our example I’ll use a four- or six-cylinder engine that has only one bank of cylinders to correct.

Assuming one had a perfect engine operating in a perfect environment, both short- and long-term fuel trims would be seen as zero. In the real world, you’ll see either a positive or negative fuel trim reading of a few percent for each and every driving condition. A slightly lean condition (a shortage of fuel or abundance of air) that just happened would be reported by the front-upstream oxygen or air-fuel sensor, and the powertrain control module would respond with a positive short trim percentage of perhaps plus-8 percent, adding fuel to bring the mixture to normal. If this were a rich condition (too much fuel or not enough air), the percentage might be minus-8 percent, reducing fuel. As the engine runs further with the condition remaining, the needed correction is learned and shifted to long-term fuel trim, allowing the short-term trim to return close to zero, awaiting the next situation that might develop.

Fuel trims typically max out at around somewhere between plus- or minus-18 percent to 30 percent before the managing system cries foul, stops further corrections, sets a diagnostic trouble code and illuminates the malfunction indicator. It’s helpful to remember that there’s a differing fuel trim for all driving conditions. Let’s say the fuel filter is somewhat clogged with debris. At low speed-load conditions, fuel flow is adequate so the fuel trims look normal. When the engine is working hard accelerating or climbing a hill and needed fuel flow exceeds delivery, the fuel trim(s) will dart sharply upward. If it’s the first time the condition has been encountered, it’ll be seen in the short-term trim. If it’s a learned-repeat condition, it’ll show up more so in the long-term fuel trim.

Multibank six-, eight-, 10-, and 12-cylinder engines often employ a separate short- and long-term fuel trim for each bank of cylinders. A malfunctioning fuel injector on the first cylinder (the first bank) would cause an unusual fuel trim reading on “bank one fuel trim,” while “bank two fuel trim” would appear normal. A dirty fuel filter or leaky mass airflow sensor hose would show equally on both bank one and bank two trims as these faults affect all cylinders.

Fuel trims provide helpful diagnostic information, but there can still be a fly or two in the ointment. Individual cylinder problems get averaged in with their siblings on that bank, and the fuel trims are only as reliable as the oxygen or air-fuel sensor that reported the issue.

Now that you’re armed and dangerous with the scan tool, remember: “If it isn’t broke, don’t fix it!”