Description
The powertrain control module (PCM) uses a Closed-Loop system in order
to control the ratio of air and of fuel. The closed-loop system 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 displays changes
in fuel delivery by indicating the values of the long term fuel trim 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. When the HO2S indicates a rich condition, the PCM reduces
the amount of fuel that is delivered. Fuel trim values below 0 percent indicate
a reduction in fuel. A fuel trim DTC can be 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 a restriction in the inlet air passage. A duct or an inlet
hose which collapses when hot or which is blocked by debris may cause a DTC
P0172 to set.
| • | Check for the correct fuel pressure. The fuel system will be rich
if fuel pressure is too high. 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
|
| • | A MAP sensor output that causes the PCM to sense a higher manifold
pressure may cause the system to go rich. By disconnecting the MAP sensor
the PCM may substitute a fixed value for the MAP sensor. If the rich condition
disappears when the MAP sensor is disconnected, substitute a known good MAP
sensor, and check the rich 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 reading of the exhaust oxygen
content. 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 voltage
in the HO2S signal circuit that causes a false rich indication. |
| • | Check the HO2S 1 and the MAP sensor for a faulty electrical connection
to the PCM. |
If a DTC P0172 cannot be duplicated, the Freeze Frame data can be useful
in determining vehicle operating conditions when the DTC was set.
