The Trainer #41: What is fuel trim?

May 1, 2015

The May 2015 edition of The Trainer reviews how the engine management system controls feed gasses and what the LTFT and STFT numbers are trying to tell you.

Pete Meier

LTFT (Long Term Fuel Trim) and STFT (Short Term Fuel Trim) is such an important data PID (Parameter Identifier) that it is among the choice few included in the standardized Global OBD II scan tool mode. It can provide you with direction when you’re attempting to solve a variety of drivability concerns, above and beyond the P0171/174 (System lean, bank 1 or 2) and P0172/175 (System rich, bank 1 or 2) Diagnostic Trouble Codes (DTCs) that may immediately come to mind.

To take advantage of all these two (or four) data PIDs can relay, though, you need to understand what they are and what they mean in terms of engine management. The first rule to remember is that the primary reason we have computer engine controls in the first place is to regulate emissions from the tailpipe. The catalytic converter is not the primary player in cleaning up exhaust emissions. In fact, it relies on a steady diet of darn near perfect (read “stoichiometric”) feed gasses or else it hurts itself. Too much air in and the converter can overheat and meltdown, too much fuel and the converter can overheat and meltdown. The Engine Control Module (ECM) has to keep the air/fuel mixture just right at all times to avoid either extreme. It does so through a feedback system operating in a closed loop.

Know what that last sentence means?

If you aren't 100% sure of how the engine management system controls those feed gasses, or what the LTFT and STFT numbers are trying to tell you, or what RFT or TFT mean, the May 2015 edition of “The Trainer” is for you!

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About the Author

Pete Meier | Creative Director, Technical | Vehicle Repair Group

Pete Meier is the former creative director, technical, for the Vehicle Repair Group with Endeavor Business Media. He is an ASE certified Master Technician with over 35 years of practical experience as a technician and educator, covering a wide variety of makes and models. He began writing for Motor Age as a contributor in 2006 and joined the magazine full-time as technical editor in 2010. Pete grew the Motor Age YouTube channel to more than 100,000 subscribers by delivering essential training videos for technicians at all levels.

Connect with Pete on LinkedIn.