OBD-II (the second generation of onboard diagnostics standards) was introduced in 1996, specifying parameters for diagnostic connectors together with a standardised format for diagnostic trouble codes (DTCs). For example, if the engine control module (ECM) detects a misfire, it must log it in the form of a “P” (powertrain) code. A P0301 means the ECM detected a misfire it thinks is from cylinder 1. The outcome, within reason, was that a diagnostic device should be able to query any compliant-vehicle onboard computer.
The OBD-II standard also introduced short- and long-term fuel trim.
This month, Autodata’s engineers break down fuel trims, including what they are, what they mean, and how to find them.
The short-term fuel trim’s purpose is to either add or subtract fuel based on how the engine is running. long-term fuel trim adds or subtracts fuel based on a trend the ECM sees.
If there is a consistent +10% of the short term, the ECM will move that over to the long term and the short term is reset to 0%. Not all vehicles follow this flow. For example, some Toyota models immediately add to the long-term trim.
What do fuel trims mean?
If you see a positive fuel trim (either short or long), the ECM has decided the engine was lacking fuel based on readings from the front (pre-catalytic converter) oxygen sensor, or air/fuel ratio sensor.
This would be a lean fuel trim. If, however, the trim is negative, the ECM is taking fuel away. This is called a rich fuel trim.
The ECM is generally looking to maintain the stoichiometric air/fuel ratio – theoretically 14.7 parts air and one-part fuel, although vehicles, in reality, typically trend leaner. This figure specifically reflects the mix at sea level and will change with altitude. In some cases, the ECM will shoot for a mixture as lean as 22:1.
How do I find the fuel trims?
One of the most common issues when trying to utilise fuel trims is that various car manufacturers call their fuel trims by different names. Technicians may wrongly assume their scanner does not cover fuel trims or the ECM does not record them.
The below guide covers examples of some of the non-standard terminology used by different manufacturers.
| Manufacturer | Short-Term Trim | Long-Term Trim | Notes |
|---|---|---|---|
| Subaru | A/F Correction | A/F Learn | |
| Honda | ST FT | LT FT | Uses decimals where 0.90 is -10%, and 1.10 is +10% |
| Nissan | Combined value: ‘Alpha’ | Combined value: ‘Alpha’ | Uses decimals where 100 is +0% |
Fuel trim versus lambda
Vehicles from the European parc may display a lambda value. Lambda 1.00 indicates an air/fuel ratio of 14.7/1. Lower readings mean a lower amount of air per unit of fuel (i.e., rich.) Higher readings mean more air per fuel or, in other words, lean; confusingly, the opposite to fuel trim! To see traditional short- and long-term fuel trims on a vehicle using non-standard or lambda values, access the OBD generic without specifying a manufacturer.
Easily look up fuel trim DTCs with Autodata Diagnostic & Repair
Autodata’s Diagnostic Trouble Code lookup module contains faults and fixes relating to fuel trims, including at-a-glance component location and wiring diagrams pertaining to the ECM, oxygen sensors, and more. Search for the code or keywords from the issue to find Original Equipment-verified fixes.
Customers currently using Autodata Service & Maintenance can upgrade to Diagnostic & Repair for as little as £35 extra per month. For more information on upgrading your package call +44 (0)330 0538999.
