Circuit Description
To provide the best possible combination of driveability, fuel economy, and emission control, a closed loop air/fuel metering system is used. While in closed loop, the powertrain control module (PCM) monitors the HO2S 1 signal and adjusts the fuel delivery based upon the HO2S signal voltage. A change made to fuel delivery is indicated by the long and short term fuel trim values which may be monitored with a scan tool. Ideal fuel trim values are around 0%. If the HO2S signals are indicating a lean condition the PCM adds fuel, resulting in fuel trim values above 0%. If a rich condition is detected, the fuel trim values are below 0%, indicating that the PCM is reducing the amount of fuel delivered. If an excessively rich condition is detected, the PCM sets DTC P0172.
The PCM's maximum authority to control long term fuel trim allows a range between -23% and +16%. The PCM's maximum authority to control short term fuel trim allows a range between -11% and +20%. The PCM monitors fuel trim under various operating conditions (fuel trim cells) before determining the status of the fuel trim diagnostic. The fuel trim cells are weighted (assigned a percentage of importance) as follows:
| • | Idle, purge commanded (Cell 10 - approximately 10%). |
| • | Deceleration, purge commanded (Cell 1 - approximately 15%). |
| • | Normal driving, purge commanded (Cell 2 - approximately 25%). |
| • | Normal driving, no purge commanded (Cell 6 - approximately 45%). |
| • | Idle, no purge commanded (Cell 9 - approximately 5%). |
The vehicle may have to be operated in all of the above conditions before the PCM determines fuel trim status. The amount that fuel trim deviates from 0% in each cell and the amount that each cell is weighted directly affects whether or not the vehicle must be operated in all of the cells described above to complete the diagnostic.
Conditions for Running the DTC
| • | None of the following DTCs are present: |
| • | Throttle position (TP) sensor DTCs |
| • | Evaporative emissions (EVAP) DTCs |
| • | Intake air temperature (IAT) sensor DTCs |
| • | Manifold absolute pressure (MAP) sensor DTCs |
| • | Exhaust gas recirculation (EGR) pintle position DTCs |
| • | Engine coolant temperature (ECT) sensor DTCs |
| • | Crankshaft position (CKP) sensor DTCs |
| • | Camshaft position (CMP) sensor DTCs |
| • | Vehicle speed sensor (VSS) DTCs |
| • | Heated oxygen sensor (HO2S) DTCs |
| • | Mass air flow (MAF) sensor DTCs |
| • | Misfire DTCs |
| • | IC/Bypass DTCs |
| • | Fuel trim DTCs |
| • | Fuel injector circuit DTCs |
| • | The ECT is between 20°C (68°F) and 110°C (230°F). |
| • | The IAT is between -18°C (0°F) and 70°C (158°F). |
| • | The BARO is more than 70 kPa. |
| • | The MAP is between 15 kPa and 85 kPa. |
| • | The throttle angle is less than 95%. |
| • | The vehicle speed is less than 82 mph. |
| • | The MAF is between 3 g/s and 150 g/s. |
| • | The engine speed between 600 and 4000 RPM. |
Conditions for Setting the DTC
| • | The long term fuel trim is at or near the maximum authority of -23%. |
| • | The short term fuel trim is at or near the maximum authority of -11%. |
| • | All of the conditions are met in Fuel Trim Cells 1, 2, 5, 6, 9 and/or 10. |
Action Taken When the DTC Sets
| • | The PCM illuminates the malfunction indicator lamp (MIL) during the second consecutive trip in which the diagnostic test has run and failed. |
| • | The PCM stores conditions which were present when the DTC set as Freeze Frame and Fail Records data. |
Conditions for Clearing the MIL/DTC
| • | The PCM turns the MIL off during the third consecutive trip in which the diagnostic has run and passed. |
| • | The history DTC clears after 40 consecutive warm-up cycles have occurred without a malfunction. |
| • | The DTC may be cleared by using the scan tool Clear Info function or by disconnecting the PCM battery feed. |
Diagnostic Aids
Inspect or test the following items:
| • | Test the fuel pressure. The system goes lean if the pressure is too low. The PCM may compensate for some decrease. However, if fuel pressure is too low, a DTC P0171 may be set. |
| Refer to Fuel System Pressure Test . |
| • | Inspect the EVAP canister for fuel saturation. If the canister is full of fuel, visually and physically inspect the EVAP and fuel system components for damage. |
| • | Disconnect the MAF sensor and note if the rich condition is corrected. If the condition is corrected by disconnecting the sensor, replace the MAF sensor. Refer to Mass Airflow Sensor Replacement . |
| • | Test the fuel pressure regulator. Test for a leaking fuel pressure regulator diaphragm by inspecting the vacuum line to the regulator for the presence of fuel. If fuel is found, replace the fuel pressure regulator. Refer to Fuel Pressure Regulator Replacement . |
| • | An intermittent TP sensor output causes the system to go rich due to a false indication of the engine accelerating. The Throttle Angle displayed on a scan tool should increase steadily from 0% to 100% when opening the throttle. |
| • | Inspect for a faulty connection at the PCM. Inspect the harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and faulty terminal to wire connections. |
| • | Inspect the wiring harness for damage. If the harness appears to be OK, observe the HO2S 1 display on the scan tool while moving connectors and wiring harnesses related to the engine harness. A change in the display will indicate the location of the malfunction. |
Reviewing the Fail Records vehicle mileage since the diagnostic test last failed may help determine how often the condition that caused the DTC to be set occurs. This may assist in diagnosing the condition.
Test Description
The number(s) below refer to the step number(s) on the Diagnostic Table.
-
DTCs other than P0172 may indicate a condition present which may cause a rich condition. If this is the case, repairing the condition which caused the other DTC will most likely correct the DTC P0172.
-
If the DTC P0172 test passes while the Fail Records conditions are being duplicated, the rich condition is intermittent. Refer to Diagnostic Aids for additional information on diagnosing intermittent conditions.
Step | Action | Value(s) | Yes | No |
|---|---|---|---|---|
1 | Did you perform the Powertrain On-Board Diagnostic (OBD) System Check? | -- | ||
Are any DTCs set other than P0172? | -- | Go to applicable DTCs first. | ||
3 |
Are the displayed values more negative than the specified value? | -10% | ||
Does the scan tool indicate DTC P0172 Failed This Ign? | -- | Go to Diagnostic Aids | ||
5 | Visually and physically inspect the air filter element and replace the filter if necessary. Refer to Air Cleaner Element Replacement . Did the air filter require replacement? | -- | ||
6 | Visually and physically inspect the air intake duct for being collapsed or restricted and repair if necessary. Does the air intake duct require repair? | -- | ||
7 |
Was a problem found? | -- | ||
8 | Start the engine and note the idle quality. Is the idle low or unsteady? | -- | ||
9 | With the engine idling, use the scan tool in order to observe the Idle Air Control display. Is the IAC value less than the specified value? | 100 counts | ||
10 |
Was a problem found? | -- | ||
11 |
Was a problem found? | -- | ||
12 |
Did the fuel pressure regulator require replacement? | -- | ||
13 |
Does the TP Angle display increase through the specified range steadily and evenly from closed throttle to wide open throttle? | 0%-100% | ||
14 |
Do both values change to near the specified value? | 0% | ||
15 |
Was problem found? | -- | ||
16 |
Was a problem found? | -- | ||
17 |
Was a problem found? | -- | ||
18 |
Was a problem found? | -- | Go to Diagnostic Aids | |
19 |
Did you complete the repair? | -- | -- | |
20 | Replace the MAF sensor. Refer to Mass Airflow Sensor Replacement . Did you complete the repair? | -- | -- | |
21 |
Does the scan tool indicate DTC P0172 Failed This Ign? | -- | System OK |
