Description
The powertrain control module (PCM) uses a Closed Loop system in order
to meter fuel. This provides the best possible combination of driveability,
of fuel economy, and of emission control. The PCM monitors the signal voltage
of the heated oxygen sensor (HO2S). When the system is in Closed-Loop operation,
the PCM adjusts fuel delivery based on the HO2S signal voltage. The scan
tool will display changes in the fuel delivery by indicating the values of
the long term and of the short term fuel trim. The best fuel trim value is
around 0 percent. The PCM will add fuel when the heated oxygen sensor
signal is indicating a lean condition. Fuel trim values that are above 0 percent
indicate additional fuel. The PCM will reduce the amount of fuel that is
delivered when the HO2S indicates a rich condition. Fuel trim values below
0 percent indicate a reduction in fuel. A fuel trim DTC may set when exhaust
emissions reach excessive levels because of a lean condition or because of
a rich condition. Conditions for Running the DTC
Conditions for Running the DTC
| • | Barometric pressure is greater than 75 kPa. |
| | The fuel tank level is more than 25 percent. |
| • | Intake air temperature is between -10°C to 70°C (14°F
to 158°F). |
| • | The engine is running at a steady speed. |
| • | The engine is operating in Closed-Loop. |
Conditions for Setting the DTC
| • | The total fuel trim is more than 30 percent or equal to 30 percent
for 5 seconds. |
| • | The short term fuel trim is more than 20 percent or equal to 20
percent for 45 seconds. |
Action Taken When the DTC Sets
| • | The PCM illuminates the malfunction indicator lamp (MIL) after
two consecutive ignition cycles during which the diagnostic runs with the
fault active. |
| • | The PCM records the operating conditions at the time the diagnostic
fails. This information is stored in the Freeze Frame buffer. |
| • | The PCM enters the Fail-Safe function and stops exhaust gas recirculation
(EGR) control and evaporative emissions (EVAP) purge control. |
Conditions for Clearing the MIL/DTC
| • | The MIL turns OFF after three consecutive trips during which the
diagnostic runs and the fault is not present. |
| • | A History DTC clears after 40 consecutive warm-up cycles without
a fault. |
| • | Use the scan tool Clear Information function, or disconnect the
PCM battery feed in order to clear the DTC. |
Diagnostic Aids
Check for any of the following conditions:
| • | Check for vacuum leaks. |
| • | Check for exhaust leaks. |
| • | Check for the correct fuel pressure. The fuel system will be lean
if fuel pressure is too low. In order to determine the cause of a DTC P0171,
it may be necessary to monitor the fuel pressure while driving the vehicle
at various road speeds. Refer to
Fuel System Diagnosis
. |
| • | Check for fuel contamination. Small amounts of water can be delivered
to the fuel injectors and cause a lean exhaust indication. Too much alcohol
in the fuel may also cause a lean exhaust indication. Refer to
Alcohol/Contaminants-in-Fuel Diagnosis |
| • | A MAP sensor output that causes the PCM to sense a lower manifold
pressure may cause the system to go lean. By disconnecting the MAP sensor,
the PCM may substitute a fixed value for the MAP sensor. If the lean condition
disappears when you disconnect the MAP sensor, substitute a known good MAP
sensor, and check the lean condition again. |
| • | Check for a damaged wiring harness. Inspect the wiring harness
for damage. If the harness appears to be OK, observe the HO2S 1 display on
the scan tool while moving the connectors and the wiring harnesses related
to the sensor. A change in the display will indicate the location of the
fault. |
| • | Check for a contaminated oxygen sensor. Silicone and other contaminants
can cause the oxygen sensor to provide false lean exhaust readings. Before
replacing the HO2S, determine the cause of the contamination, and correct
the condition. |
| • | Inspect the HO2S 1 for water intrusion into the wiring harness
and into the sensor housing. Water can create a short to ground in the HO2S
signal circuit that causes a false lean indication. Check the HO2S 1 and the
MAP sensor for a faulty electrical connection to the PCM. If a DTC P0171
cannot be duplicated, the Freeze Frame data can be useful in determining
vehicle operating conditions when the DTC was set. |
