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 fueling. 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. LT and ST fuel trim can be read by using a scan tool.

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

Conditions for setting DTC:

• No IAC DTCs set at idle.
• No ECT, EGR, HO2S, IAT, MAF, MAP or TP sensor DTCs set.
• No EVAP DTCs set.
• No misfire or injector circuit DTCs set.
• Engine speed 500-4000 RPM.
• ECT 68-230°F (20-110°C).
• IAT 0-104°F (-18 to 40°C).
• BARO greater than 10.7 psi (0.75 kg/cm² ).
• MAP 2.6-12.8 psi (0.18-0.90 kg/cm² )
• TP angle less than 90 percent.
• MAF 9-180 grams per second.
• Long term fuel trim about maximum authority (16).
• Short term fuel trim about maximum authority (20).
• Vehicle speed less than 70 MPH.
• Conditions met in FUEL TRIM CELLS 0, 1, 2, 6, and/or 7.