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.

For duplication of DTC, ensure:

• No EVAP system, EST, HO2S, fuel injector circuit, EGR, misfire, idle speed, vehicle speed, MAF, ECT, TP, CKP, CMP, MAP or IAT sensor DTCs are set.
• ECT is 68-230°F (20-110°C).
• IAT is 0-158°F (-18 to 70°C).
• BARO is greater than 70 kPa.
• MAP is 2.6-11.6 psi (18-80 kPa).
• TP angle is less than 90 percent.
• Vehicle speed is less than 70 MPH.
• MAF is 3-150 grams per second.
• Engine speed is 600-4000 RPM.