Tech Tips from Identifix: What are ‘fuel trims’ & how do you find them?

Every month we source useful information and top techniques from our team of L1 Certified Master Technicians that man our Virtual Tech Hotline.

This month, our Master Tech — Joe Masterman — provides a guide on fuel trims, what they are, what they tell you, and how to find them.

What are fuel trims?

Let’s start at the beginning — 1996 — when OBD2 was implemented across the entire car industry. They had some rules, some basic operating parameters. For example, the Engine Control Module must be able to tell if the engine is misfiring, if it does detect a misfire it needs to be able to tell the technician about the misfire in the form of a “P” (powertrain) code.

A P0301 means the Engine Control Module detected a misfire that it thinks is from cylinder 1. This doesn’t mean the Engine Control Module is correct though… *cough* Honda *cough*…

OBD2 also added another parameter for the poor old Engine Control Module to keep track of, the Fuel Trim. There were two types introduced, the Short Term Fuel Trim, and the Long Term Fuel Trim. The Short Term Fuel Trim’s job is to either add or subtract fuel based on how the engine was currently running, the Long Term Fuel Trim adds or subtracts fuel based on a trend that the Engine Control Module (ECM) sees.

If there is a consistent +10% on the Short Term, the ECM will move that over to the Long Term, this way the Short Term can go back down to 0%.  This is just a general idea on the operation of the trims, and not all vehicles will behave this way, some like to add to the Long Term right off the bat, looking at you Toyota.  You may be asking, but what about “post catalyst fuel trims” and “single cylinder fuel trims”? We’ll get to those in another article.

What do fuel trims mean?

If you see a fuel trim (either Short or Long Term) that is positive, let’s say +10%, then the ECM has decided the engine was lacking enough fuel based on the readings from the front (pre-catalytic converter) Oxygen Sensor (sometimes an Air/Fuel Ratio Sensor, but that’s also a whole other article). This would be a Lean fuel trim.

If, however, the trims are -10% then the ECM is taking fuel away, this is a Rich fuel trim. The ECM is always hunting for that elusive stoichiometric air/fuel ratio, this is theoretically 14.7 parts air and 1 part fuel, although vehicles typically trend a bit leaner than that in reality. In some cases, the ECM will shoot for a mixture as lean at 22:1 Yowsa!

How do I find fuel trims?

“But I can’t find fuel trims!”  you say, “My scanner doesn’t do that!” you may wrongly assume, “Tell me something else to check instead!” One of the most common issues when trying to utilize fuel trims is that or techs don’t recognize their names or their normal function in the data menu.

Various car manufacturers call their fuel trims by different names. Subaru, for example, calls their Short Term Fuel Trims “A/F Correction”, and their Long Term is “A/F Learn”. Honda is sometimes generous enough to include verbiage like “ST FT” for Short Term Fuel Trim, but instead of the usual + or – 10% readings, we expect we see 1.00, or .90, or 1.10.

For a Honda 1.00 is stoichiometric or 0%.  While a .9 reading is the same as -10%, and a 1.10 reading is the same as +10%. Nissan likes to do away with the Short and Long terms completely and compile them into one reading, called Alpha. Alpha behaves a bit like the Honda trims, but it starts at 100 rather than 1.00, just ignore the decimal point.

FUN, INTERESTING & COOL AUTO FACTS: http://bit.ly/FunInterestingCoolAutoFacts

Some vehicles, for example, every European vehicle on the planet, like to display a “Lambda” Reading. Lambda starts at 1.00, this indicates an Air/Fuel ratio of 14.7/1, lower readings mean lower amounts of Air per 1 Fuel… rich. While higher readings mean more Air per Fuel… lean. You can see how easy it is to be confused.

Unless you are a practiced drivability tech you may be lost in the weeds a bit.  If you find yourself swimming in a sea of unusual verbiage just go into OBD Generic with your scan tool, don’t tell the scan tool you’re working on an Infiniti or Porsche, and you will see your old familiar Short Term and Long Term Fuel Trims. This is a mandate per OBD2, it MUST be there!

For a little extra information, we’ve also compiled a little cheat sheet that lists the terminology manufactures like to use for fuel trims.  Feel free to download it, print it out, and put it behind a pane of glass with the label “Break In Case Of Vehicle Fuel Trim Diag!” It’s a bit wordy, I know, but we don’t want to go breaking glass for no reason.

DOWNLOAD THE FUEL TRIM NAMING CHEAT SHEET