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 PCM monitors the HO2S 1 signal and adjusts fuel delivery based upon the HO2S signal voltage. A change made to fuel delivery will be indicated by the long and short term fuel trim values which can be monitored with a scan tool. Ideal fuel trim values are around 0 percent; if the HO2S signals are indicating a lean condition the PCM will add fuel, resulting in fuel trim values above 0 percent. If a rich condition is detected, the fuel trim values will be below 0%, indicating that the PCM is reducing the amount of fuel delivered. If an excessively rich condition is detected, the PCM will set DTC P0172.
The PCM's maximum authority to control long term fuel trim allows a range between -23 percent and +16 percent. The PCM's maximum authority to control short term fuel trim allows a range between -11 percent and +20 percent. 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 in determining total fuel trim status for the diagnostic) as follows:
| • | Idle, purge commanded (Cell 10 - approximately 10 percent). |
| • | Deceleration, purge commanded (Cell 1- approximately 15 percent). |
| • | Normal driving, purge commanded (Cell 2- approximately 25 percent). |
| • | Normal driving, no purge commanded (Cell 6- approximately 45 percent). |
| • | Idle, no purge commanded (Cell 9- approximately 5 percent). |
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 percent 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
| • | No VSS, TP sensor, EVAP system, HO2S, Misfire, IAT sensor, MAP sensor, Idle speed, Fuel Injector(s), IC/Bypass, EGR Pintle, EGR Flow, ECT sensor CKP sensor CMP sensor, or MAF sensor DTC(s) set. |
| • | Engine coolant temperature between 20°C (68°F) and 110°C (230°F). |
| • | Intake air temperature is between -18°C (0°F) and 70°C (158°F). |
| • | BARO is more than 70 kPa. |
| • | Manifold absolute pressure is between 15 kPa and 85 kPa. |
| • | Throttle angle is less than 90 percent. |
| • | Vehicle speed is less than 82 mph. |
| • | Mass air flow is between 3 g/s and 150 g/s. |
| • | Engine speed between 600 and 4000 RPM. |
Conditions for Setting the DTC
| • | Long term fuel trim is at or near maximum authority of -23 percent. |
| • | Short term fuel trim is at or near maximum authority of -11 percent. |
| • | All conditions have been met in Fuel Trim Cells 1, 2, 6, 9 and/or 10. |
Action Taken When the DTC Sets
| • | The PCM will illuminate the MIL during the second consecutive trip in which the diagnostic test has been run and failed. |
| • | The PCM will store conditions which were present when the DTC set as Freeze Frame and Fail Records data. |
Conditions for Clearing the MIL/DTC
| • | The PCM will turn the MIL OFF during the third consecutive trip in which the diagnostic has been run and passed. |
| • | The history DTC will clear after 40 consecutive warm-up cycles have occurred without a malfunction. |
| • | The DTC can be cleared by using a scan tool. |
Diagnostic Aids
Check the following items:
| • | Fuel pressure: The system will go lean if pressure is too low. The PCM can compensate for some decrease. However, if fuel pressure is too low, a DTC P0171 may be set. |
| Refer to Fuel System Pressure Test . |
| • | EVAP Canister: Check the EVAP canister for fuel saturation. If the canister is full of fuel, visually and physically check the EVAP and fuel system components for damage. |
| • | Mass Air Flow Sensor: Disconnect the MAF sensor and see if rich condition is corrected. If so, replace the MAF sensor. Refer to Mass Airflow Sensor Replacement . |
| • | Fuel Pressure Regulator: Check for a leaking fuel pressure regulator diaphragm by checking vacuum line to regulator for the presence of fuel. If a problem is found, replace the fuel pressure regulator. Refer to Fuel Pressure Regulator Replacement . |
| • | TP Sensor: An intermittent TP sensor output will cause the system to go rich due to a false indication of the engine accelerating. Throttle Angle displayed on a scan tool should increase steadily from 0 percent to 100 percent when opening the throttle. If not, check the TP sensor for loose mounting screws. If the TP mounting screws are OK, replace the TP sensor. Refer to Throttle Position Sensor Replacement . |
| • | Poor connection at the PCM: Inspect harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and poor terminal to wire connection. |
| • | Damaged 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 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 numbers below refer to the step numbers 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.
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? | -- | Diagnose the other DTCs first. | ||
3 |
Are the displayed values more negative than the specified value? | -10% | ||
Does the scan tool indicate DTC P0172 failed this ignition? | -- | Go to Diagnostic Aids | ||
5 | Visually and physically inspect the air filter element and replace it 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. Did your inspection reveal a condition requiring repair? | -- | ||
7 |
Refer to Throttle Body Assembly Replacement . Was a problem found? | -- | ||
8 | Start the engine and note the idle quality. Is a low or unsteady idle being experienced? | -- | ||
9 | With the engine idling, observe the Idle Air Control display on the scan tool. Is Idle Air Control value less than the specified value? (Value is displayed in counts). | 100 | ||
10 |
Refer to Fuel System Cleaning . Was a problem found? | -- | ||
11 |
Was a problem found? | -- | ||
12 |
Did the fuel pressure regulator require replacement? | -- | ||
13 |
Does the TP Angle display increase steadily and evenly from minimum value at closed throttle to maximum value at wide open throttle? | 0%-100% | ||
14 |
Did 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 ignition? | -- | System OK |
