Circuit Description

PCM reads HO2S signal voltage and adjusts fuel delivery based on this voltage. A change made to fuel delivery is indicated by Long Term (LT) and Short Term (ST) FUEL TRIM values.

ST FUEL TRIM values change rapidly in response to HO2S signal voltages. These changes fine tune engine fuel delivery. LT FUEL TRIM values change in response to trends in ST fuel trim. LT fuel trim makes coarse adjustments to fueling in order to re-center and restore control to ST fuel trim.

Ideal FUEL TRIM value is about zero. Fuel trim value more than zero indicates PCM is adding fuel to compensate for a lean condition. Fuel trim less than zero indicates PCM is reducing amount of fuel to compensate for rich condition. DTC will set if PCM detects an excessively lean condition.

Conditions required to set DTC are:

• No scan tool test is being run.
• No emission related DTCs set.
• ECT is 149-219°F (65-104°C).
• IAT is -40-248°F (-40-120°C).
• MAP sensor values are between 23.75-99 kPa.
• Barometric pressue is more than 72.3 kPa.
• Engine speed is 400-6000 RPM.
• System voltage is greater than 9.5 volts.
• Engine is operating in closed loop.
• Average short term fuel trim is greater than .97.
• Adaptive index multiplier samples are greater than 1.21.