The front oxygen sensor sends signals on exhaust gas oxygen content to the PCM
to control a 14.7:1 air/fuel ratio under normal driving conditions. The PCM can make
fuel corrections from a nominal 128 (0% correction) short/long term fuel trim
value to a maximum window of 101-147 (-21 to +15 percent). If the
PCM determines a rich condition exists, 02 voltage above 450 mV, it will decrease
injector pulse width, reducing fuel, to maintain a 14.7:1 air/fuel ratio. The short
term fuel trim value will be between 101-128 indicating a rich condition.
If the PCM determines a lean condition exists, 02 voltage below 450 mV, it
will increase injector pulse width, adding fuel, maintaining a correct air/fuel mixture.
The short term fuel trim value will be between 128-147 indicating a lean condition.
When certain conditions have been met to begin learning fuel control (ECT, closed
loop, etc.) the PCM will use the short term fuel trim cell to update 1 of the 4 long
term cells (idle, decel, cruise, accel) being used. During every drive cycle, or trip,
the PCM will use and update the idle and cruise long term cells and will keep track
of how long the control system is operating in these cells. After approximately 2 minutes
of being in each of these cells, the fuel control system will enable EVAP purge and
the purge long term cells will be used. Refer to EVAP Canister Purge for information
on purge long term fuel cells. If the average of the idle and cruise fuel cells is
147 and the short term fuel trim cell is currently over 128, a lean fuel system DTC
will set. This test is performed before the purge cells are used.
Conditions for Setting the DTC
Important: The fuel control system is continuously checked
when the following conditions have been met.
DTC P0171 will set if the average long term idle and cruise fuel trim cells
are 147 and the current short term fuel trim cell is above 128 for 3 seconds
when:
| • | The PCM is using the non-purge long term fuel trim cells. |
| • | The system is in closed loop. |
| • | The engine coolant is between 60-115°C (140-239°F). |
| • | Air flow is between 2-80 grams/second. |
| • | Manifold pressure is between 20-90 kPa (3-13 psi). |
| • | The RPM is between 500-4,000 RPM. |
| • | The TP is less than 75 percent open. |
| • | The barometer is greater than 70 kPa (10 psi). |
| • | The IAT reading is between -30 to +80°C (-22 to +176°F). |
| • | The vehicle speed is less than 121 km/h (75 mph). |
| • | No ECT, MAP, IAT, TP sensor, VSS, EVAP canister purge, front O2S, or P0125
DTCs have been set. |
Possible Causes
Important: If the front oxygen reading voltage is below
300 mV and the rear HO2S-2 is below 300 mV all the time, the O2 sensors
are probably not at fault.
The following are malfunctions that could cause a lean condition:
| • | Fuel pressure too low -- Refer to Engine/Fuel/Air Intake/Exhaust
Service. |
| • | Exhaust leakage around the manifold or front O2S sensor |
