Scan tool has short, long-term fuel trims


I just bought a scan tool online and am testing my ‘98 Civic. It’s great checking all the things it can do. Can you explain what “fuel trims” are and how I could know they’re OK?

– Adam Welch, Santa Clara, Calif.

Great question, and one we deal with regularly in my engine-performance class. I challenged one of my very talented students, Chris Nuanez, to tackle your question. Here’s Chris’ response:

This is an excellent feature of a scan tool. The PCM (powertrain control module) is allowing you to see how it corrects for the air/fuel ratio being too lean or too rich. This is done by observing the oxygen sensor’s activity and making corrections based on the voltages it is producing.

There are two fuel trims that are displayed on the scan tool: short term (STFT), and long term (LTFT). They can be displayed in different ways. In the case of Honda, the trims are displayed as 1.00, and the corrections are seen as a value above or below this number, 0.99, 0.98, 0.97, etc., or 1.01, 1.02, 1.03, etc.

The 1.00 value indicates that the system is at the optimum air/fuel ratio and requires no correction. If the system is running too lean, the computer will correct for this by increasing the amount of fuel, lengthening pulse width of the fuel injectors, and a value above 1.00 will be seen.

If the system is too rich, the computer will reduce fuel delivery, and a value less than 1.00 will be seen on the scan tool.

Other car manufacturers might use a trim value of 100 percent, and a value above or below this – plus/minus percent – indicates which way the computer is correcting. A value within 5 percent of 1.00 or 100 percent is desirable.

Short-term fuel trim is an immediate correction. The computer is watching the oxygen sensor to see how much oxygen is in the exhaust. If it sees too much oxygen, the air/fuel mixture is too lean and it will drive the system rich: lean condition-rich command.

If there is not enough oxygen, the mixture is too rich and it will drive the system lean: rich condition-lean command.

Long-term fuel trim assists with continuous issues. If the problem with the fuel system persists for a predetermined period, the computer will shift the correction from short term to long term. Shifting a persisting problem over to the long term will give the short term more room to make immediate corrections.

However, the computer can correct a problem only to a certain extent. If the problem is severe, requiring a correction above 25 percent, (in the case of a Honda, it’s displayed as 0.75 or 1.25), the computer cannot correct any further and the malfunction indicator lamp will light.

Observing the short- and long-term fuel trims can tell you if there is a problem and which way the PCM is correcting for it. This can give technicians a good heads-up on what is going on and point them in the right direction on how to further diagnose a problem.

Brad Bergholdt is an automotive technology instructor at Evergreen Valley College in San Jose.