Short/Long Term Fuel Trim (ST/LT Trim)

^ "Short Term" Fuel Trim is the value that represents the quick small changes the PCM makes to the A/F ratio.

^ On a Sonata, the PCM continuously responds to the signals from the oxygen sensors to maintain the 14.7 to 1 A/F ratio by increasing or decreasing fuel delivery.

^ On Accent, Elantra and Tiburon, the "Short Term" Fuel Trim indicates the amount of correction at idle and low rpm/load.

^ "Short Term" Fuel Trim values range from -25% to +25%.

^ "Short Term" Fuel Trim values are positive when the PCM is increasing fuel delivery, negative when the PCM is decreasing fuel delivery. "Short Term" Fuel Trim may read "0.0" for brief periods during transition from positive to negative (or negative to positive) operation. If the "Short Term" Fuel Trim reads "0.0" for longer periods, it may indicate that the vehicle is in open loop.

^ On a Sonata, the "Short Term" Fuel Trim values are not retained in the PCM memory when the ignition is turned off.

^ On a Sonata, "Long Term" Fuel Trim values change to compensate when the "Short Term" Fuel Trim has reached its limit in one direction and is unable to maintain the 14.7 to 1 A/F ratio.

^ On Accent, Elantra and Tiburon vehicles, the "Long Term" Fuel Trim indicates the amount of correction at high rpm/load.

^ The "Long Term" Fuel Trim correction range is -25% to +25%. Exceeding the -20% to +20% range will set a code on an Accent, Elantra and Tiburon. Sonata vehicles will set a code when the -12.5% to +12.5% range is exceeded.

^ On a Sonata, a one digit change of the "Long Term" Fuel Trim value is roughly the equivalent of one entire "Short Term" Fuel Trim range. For example: when the "Short Term" Fuel Trim reaches its limit, the "Long Term" Fuel Trim moves one digit. This process repeats until the A/F ratio returns to 14.7 to 1 or the "Long Term" Fuel Trim correction range limit has been exceeded.

^ There is a preprogrammed "Long Term" Fuel Trim correction range that is acceptable for normal operation. When the "Long Term" Fuel Trim exceeds this range in two driving cycles, the vehicle will set a diagnostic code (DTC), which will illuminate the check engine light.

^ "Long Term" Fuel Trim values are retained in the PCM's memory when the ignition is turned off. This allows the vehicle to "learn" a new fuel delivery schedule, which is "fine tuned" for optimal performance.