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 more 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 required to set DTC are:

• No IAC DTCs set at idle.
• No CKP, ECT, EGR, HO2S, IAT, MAF, MAP or TP sensor DTCs set.
• No EVAP DTCs set.
• No misfire or injector circuit DTCs set.
• No scan tool test is being run.
• ECT 122-212°F (50-100°C).
• IAT -40-248°F (-40-120°C).
• BARO more than 72.5 kPa.
• Engine speed 400-600 RPM.
• MAP is between 20-99 kPa
• fuel system is in closed loop.
• If canister purge is on, purge cycle is greater than zero percent.
• TP ANGLE less than 95%.
• MAF 2-200 grams per second.
• Long term fuel trim above 16 percent.
• Vehicle speed is less than 85 MPH (136 km/h).
• Short term fuel trim above 20 percent.