Circuit Description

PCM monitors oxygen sensor signal voltage and adjusts fuel delivery based on signal voltage. A change made to fuel delivery is indicated by long and short term fuel trim values.

Short term fuel trim values change rapidly in response to HO2S signal voltages. These changes fine tune engine fueling. Long term fuel trim values changes in response to trends in short term fuel trim. Long term fuel trim makes coarse adjustments to fueling in order to re-center and restore control to short term fuel trim. Long term and short term fuel trim can be monitored by use of scan tool.

Ideal fuel trim value is about zero percent. A positive fuel trim indicates that PCM is adding fuel to compensate for a lean condition. A negative fuel trim indicates that PCM is reducing amount of fuel to compensate for a rich condition. DTC will set if PCM detects an excessively rich or lean condition.

DTC will set when following conditions are present:

• DTCs P0100, P0101, P0102, P0103, P0107, P0108, P0112, P0113, P0117, P0118, P0120, P0121, P0122, P0123, P0125, P0131, P0132, P0134, P0135, P0151, P0152, P0154, P0155, P0200, P0220, P0221, P0225, P0226, P0300, P0335, P0336, P0351-P0358, P0372, P0410, P0412, P0418, P0430, P0441, P0443, P0500, P0501, P0503, P0601, P1111, P1112, P1120, P1121, P1125, P1133, P1153, P1220, P1221, P1258, P1351, P1361, P1371, P1415, P1416, P1441, P1514, P1515, P1516 or P1517 not set.
• Engine coolant temperature 120-239°F (60-115°C).
• Fuel system operating in closed loop.
• Fuel trim enabled.
• MAP 20-90 kPa.
• EVAP canister purge not enabled or EVAP canister purge duty cycle more than 75 percent.
• PCM detects average long term fuel trim cell values are more than a predetermined threshold.
• BARO more than 70 kPa.
• MAF 5-100 gm/s.
• IAT -22 to 302°F (-22 to 150°C).
• Engine speed 500-4000 RPM.
• TP sensor angle less than 75 percent.
• Vehicle speed less than 75MPH.
• Conditions met for 10 seconds.