For 1990-2009 cars

The short term fuel trim is used by the powertrain control module (PCM) in order to control the fuel injector pulse width (injector ON time). When the fuel metering control system is in closed loop operation, the PCM monitors the fuel trim oxygen sensor signal, in order to determine if the fuel system is operating rich or lean. If the PCM determines that no change in fuel delivery is needed in order to maintain the correct air/fuel ratio for a particular engine operating condition, a short term fuel trim scan tool data value of 0 percent (128 counts) is indicated. If the control module determines that the fuel system is operating rich, short term fuel trim will compensate for the rich condition by decreasing the fuel injector pulse width. When this happens the scan tool will indicate a short term fuel trim value of less than 0 percent (128 counts). If the PCM determines that the fuel system is operating lean, short term fuel trim will compensate for the lean condition by increasing the fuel injector pulse width. In this case the scan tool will indicate a short term fuel trim value of more than 0 percent (128 counts).

#### Short Term Fuel Trim Operation

Oxygen Sensor Indication | Fuel Control Response | Short Term Fuel Trim - Percent | Short Term Fuel Trim - Counts
--- | --- | --- | ---
Lean Mixture | Increase Fuel | 1% to 30% | Greater than 128 Counts
Correct Mixture | No Change | 0% | 128 Counts
Rich Mixture | Decrease Fuel | -1% to -30% | Less than 128 Counts

When the PCM is in the power enrichment mode, the short term fuel trim value will indicate 0 percent (128 counts). This prevents the fuel trim from compensating for the temporary rich command of the power enrichment mode.