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%; if the HO2S signals are indicating a lean condition the PCM will add fuel, resulting in fuel trim values above 0%. 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% 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 used to determine fuel trim are as follows:
| • | Idle, purge commanded (Cell 10). |
| • | Deceleration, purge commanded (Cell 1). |
| • | Normal driving, purge commanded (Cell 2). |
| • | Deceleration, no purge commanded (Cell 5). |
| • | Normal driving, no purge commanded (Cell 6). |
| • | Idle, no purge commanded (Cell 9). |
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
| • | No active fuel trim, fuel injector circuit, Ignition Control circuit, Misfire, Idle speed, EVAP, HO2S, EGR, AIR, VSS, TP, IAT, MAP, ECT, CKP, CMP or MAF sensor DTCs present. |
| • | ECT between 20°C (68°F) and 110°C (230°F). |
| • | IAT is between -18°C (0°F) and 70°C (158°F). |
| • | BARO is greater than 70 kPa. |
| • | MAP is between 15 kPa and 85 kPa. |
| • | Throttle angle is less than 50%. |
| • | Vehicle speed is less than 83 mph. |
| • | MAF 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%. |
| • | Short term fuel trim is at or near maximum authority of -11%. |
| • | All conditions have been 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) on the second consecutive ignition cycle that the diagnostic runs and fails. |
| • | The PCM records the operating conditions at the time the diagnostic fails. The first time the diagnostic fails, the PCM stores this information in the Failure Records. If the diagnostic reports a failure on the second consecutive ignition cycle, the PCM records the operating conditions at the time of the failure. The PCM writes the conditions to the Freeze Frame and updates the Failure Records. |
Conditions for Clearing the MIL/DTC
| • | The PCM turns the MIL Off after three consecutive drive trips that the diagnostic runs and does not fail. |
| • | A last test failed (current DTC) clears when the diagnostic runs and does not fail. |
| • | A History DTC clears after forty consecutive warm-up cycles, if this or any other emission related diagnostic does not report any failures. |
| • | Use a scan tool in order to clear the MIL/DTC. |
| • | Interrupting the PCM battery voltage may or may not clear DTCs. This practice is not recommended. Refer to Powertrain Control Module (PCM) Description , Clearing Diagnostic Trouble Codes. |
Diagnostic Aids
Notice: Use the connector test adapter kit J 35616-A for any test that
requires probing the following items:
• The PCM harness connectors • The electrical center fuse/relay cavities • The component terminals • The component harness connector
Check the following items:
| • | Fuel pressure: The system may 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 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. |
| Refer to EVAP Control System Diagnosis. |
| • | 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% to 100% 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 . |
Reviewing the Failure 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.
If the problem is intermittent, refer to Intermittent Conditions .
Test Description
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 the applicable DTC table. | ||
3 |
Are the displayed values more negative than the specified value? | -10% | ||
Does the scan tool indicate that this test ran and passed? | -- | Refer to Diagnostic Aids | ||
5 | Visually and physically inspect the air filter element and replace it if necessary. Was a problem found and corrected? | -- | ||
6 | Visually and physically inspect the air intake duct for being collapsed or restricted. Was a problem found and corrected? | -- | ||
7 | Inspect the throttle body, duct work and the MAF sensor for damage or for the presence of foreign objects which may alter the airflow sample through the MAF sensor. Was a problem found and corrected? | -- | ||
8 | Start the engine and note the idle quality. Is a low or unsteady idle being experienced? | -- | ||
9 | With the engine idling, observe IAC Position on the scan tool. Is IAC Position less than the specified value? (Value is displayed in counts). | 100 | ||
10 | Physically inspect the throttle body bore, throttle plate, and IAC passages for coking and foreign objects. Was a problem found and corrected? | -- | ||
11 |
Was a problem found? | -- | ||
12 |
Was a problem found and corrected? | -- | ||
13 | Monitor the TP Angle display on the scan tool while slowly depressing the accelerator pedal. 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 | Perform the Fuel System Pressure Test. Refer to Fuel System Pressure Test . Was problem found and corrected? | -- | ||
16 | Perform the Evaporative Emissions Control (EVAP) System service bay test. Refer to DTC P0440 Evaporative Emission (EVAP) System . Was a problem found and corrected? | -- | ||
17 | Check fuel injector operation. Refer to Fuel Injector Balance Test . Was a problem found and corrected? | -- | ||
18 |
Was a problem found and corrected? | -- | Refer to Diagnostic Aids. | |
19 |
Is the action complete? | -- | -- | |
20 | Replace the MAF sensor. Refer to Mass Airflow Sensor Replacement . Is the action complete? | -- | -- | |
21 |
Does the scan tool indicate that this test ran and passed? | -- | ||
22 | Review Captured Info using the scan tool. Are there any DTCs that have not been diagnosed? | -- | Go to the applicable DTC table | System OK |
