Circuit Description

WARNING: This page is about a different car, the 2000 Isuzu Hombre. However, it is still accessible from the selected car via links, so may be relevant.

Vehicle Control Module (VCM) reads Heated Oxygen Sensor (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 128. Fuel trim value greater than 128 indicates that VCM is adding fuel to compensate for a lean condition. Fuel trim less than 128 indicates that VCM is reducing amount of fuel to compensate for rich condition. DTC will set if VCM detects an excessively rich or lean condition.

Conditions required to set DTC are:

No IAC DTCs set (at idle)
No ECT, EGR, HO2S, IAT, MAF, MAP, TP or VSS sensor DTCs set.
No EVAP or voltage system DTCs set.
No misfire DTCs set.
TP angle less than 69.9 percent.
Engine speed is 575-4500 RPM.
BARO is greater than 70 kPa.
MAP is 22-85 kPa.
IAT is 46-169°F (8-76°C).
MAF is 3-85 gms/sec.
Vehicle speed less than 85 MPH.