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 rich condition. DTC will set if PCM detects an excessively rich or lean condition.

For duplication of DTC, ensure:

• DTCs P0101, P0102, P0103, P0107, P0108, P0112, P0113, P0117, P0118, P0121, P0122, P0123, P0125, P0300, P0335, P0336, P0351-P0358, P0401, P0404, P0405, P0410, P0412, P0418, P0443, P0500, P0502, P0503, P1111, P1112, P1258, P1404, P1415 or P1416 are not set.
• ECT is 120-239°F (60-115°C).
• BARO is greater than 74 kPa.
• MAF is 5-90 gm/s.
• MAP is 26-90 kPa.
• IAT is -4°C to 194°F (-20°C to 90°C).
• Engine speed is 400-3000 RPM.
• TP sensor angle is less than 90 percent.
• Vehicle speed is less than 85 MPH.
• Fuel level is greater than 10 percent.
• PCM detects average long term fuel trim cell values are more than a predetermined threshold.