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 fuel delivery based upon the HO2S signal voltages. A change made to fuel delivery will be indicated by the Long and Short Term FT 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 percent, 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 PCMs maximum authority to control long term fuel trim allows a range between -30 percent and 24 percent. The PCMs maximum authority to control short term fuel trim allows a range between -27 percent and 27 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 as follows:
Cell | Purge ON | Purge OFF |
|---|---|---|
Idle (Cell 0) | X | -- |
Decel (Cell 1) | X | -- |
Normal (Cell 2) | X | -- |
Accel (Cell 3) | -- | -- |
High Flow (Cell 4) | -- | -- |
Long Term FT average less than -20 percent | ||
Short Term FT less than 0 percent | ||
The vehicle may have to be operated in all of the above conditions marked by an X, before the PCM determines fuel trim status. The amount of fuel trim deviates from 0 percent in each cell and the amount that each cell is used 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 MAF, ECT, TP, MAP, CMP, CKP, IAT sensor, EVAP, EGR, HO2S, Misfire, Injector circuit, Idle speed, VSS, or IC circuit DTCs 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). |
| • | Manifold absolute pressure is between 15-85 kPa. |
| • | Throttle angle is steady and less than 90 percent. |
| • | Vehicle speed is less than 132 km/h (82 mph). |
| • | Engine speed is between 600-4000 RPM. |
| • | Baro is more than 70 kPa. |
| • | Air flow is between 3 gm/s and 150 gm/s. |
Conditions for Setting the DTC
| • | Long Term FT is at or near maximum authority of 20 percent. |
| • | Short Term FT is at or near maximum authority of 0 percent. |
| • | All conditions have been met in the fuel trim cells. |
Action Taken When the DTC Sets
| • | The PCM will illuminate the malfunction indicator lamp (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/Failure Records data. |
Conditions for Clearing the MIL/DTC
| • | The PCM will turn OFF the malfunction indicator lamp (MIL) during the third consecutive trip in which the diagnostic has 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
Inspect the following items:
| • | Fuel pressure high--The system will go rich if pressure is too high. The PCM can compensate for some increase. However, if fuel pressure is too high, a DTC P0172 may be set. Refer to Fuel System Diagnosis . |
| • | Fuel injectors faulty--Refer to Fuel Injector Balance Test . |
| • | Inspect the EVAP canister for fuel saturation-- If the EVAP canister is full of fuel, inspect canister control and hoses. |
| • | Disconnect the MAF sensor and see if rich condition is corrected. If so, replace the MAF sensor. Refer to Mass Airflow Sensor Replacement . |
Many situations may lead to an intermittent condition. Perform each inspection or test as directed.
Important: : Remove any debris from the connector surfaces before servicing a component. Inspect the connector gaskets when diagnosing or replacing a component. Ensure that the gaskets are installed correctly. The gaskets prevent contaminate intrusion.
| • | Loose terminal connection |
| - | Use a corresponding mating terminal to test for proper tension. Refer to Testing for Intermittent Conditions and Poor Connections , and to Connector Repairs in Wiring Systems for diagnosis and repair. |
| - | Inspect the harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and faulty terminal to wire connection. Refer to Testing for Intermittent Conditions and Poor Connections , and to Connector Repairs in Wiring Systems for diagnosis and repair. |
| • | Damaged harness--Inspect the wiring harness for damage. If the harness inspection does not reveal a problem, observe the display on the scan tool while moving connectors and wiring harnesses related to the sensor. A change in the scan tool display may indicate the location of the fault. Refer to Wiring Repairs in Wiring Systems for diagnosis and repair. |
| • | Inspect the powertrain control module (PCM) and the engine grounds for clean and secure connections. Refer to Wiring Repairs in Wiring Systems for diagnosis and repair. |
If the condition is determined to be intermittent, reviewing the Snapshot or Freeze Frame/Failure Records may be useful in determining when the DTC or condition was identified.
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 lean 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 Failure Records conditions are being duplicated, the rich condition is intermittent. Refer to Diagnostic Aids or Symptoms for additional information on diagnosing intermittent problems.
Step | Action | Values | Yes | No | ||||
|---|---|---|---|---|---|---|---|---|
1 | Did you perform the Powertrain On-Board Diagnostic (OBD) System Check? | -- | ||||||
Are any DTCs set other than P0172? | -- | Go to the applicable DTCs | ||||||
3 |
Are the displayed values at or near the specified values?
| -20 % -0 % | ||||||
Does the scan tool indicate DTC P0172 failed this ignition? | -- | Go to Diagnostic Aids | ||||||
5 | Turn OFF the engine. Visually and physically inspect the air filter element and replace it if necessary. Refer to Air Cleaner Assembly Replacement . Did you complete the replacement? | -- | ||||||
6 | Visually and physically inspect the air intake duct for being collapsed or restricted and repair if necessary. Did you complete the repair? | -- | ||||||
7 | Inspect the throttle body inlet screen for damage or for the presence of foreign objects which may partially block airflow through the screen and correct any problem found. Did you find and correct the condition? | -- | ||||||
8 | Start the engine and note the idle quality. Is a low or unsteady idle being experienced? | -- | ||||||
9 | With the engine idling, observe the IAC display on the scan tool. Does the IAC value measure less than the specified value? (Value is displayed in counts). | 100 counts | ||||||
10 |
Did you complete the repair? | -- | ||||||
11 |
Did you complete the repair? | -- | ||||||
12 |
Did you complete the repair? | -- | ||||||
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 |
Did you find and correct the condition? | -- | ||||||
16 |
Did you complete the repair? | -- | ||||||
17 | Perform the Injector Balance Test. Refer to Fuel Injector Balance Test . Did you complete the repair? | -- | ||||||
18 |
Did you complete the replacement? | -- | Go to Diagnostic Aids | |||||
19 | Replace the TP sensor. Refer to Throttle Position Sensor Replacement . Did you complete the replacement? | -- | -- | |||||
20 | Replace the MAF sensor. Refer to Mass Airflow Sensor Replacement . Did you complete the replacement? | -- | -- | |||||
21 |
Does the DTC reset? | -- | System OK |
