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 voltages. 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 PCMs maximum authority to control long term fuel trim allows a range between -25% and +20%. The PCMs maximum authority to control short term fuel trim allows a range between -27% and +27%. 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) | -- | -- |
Accel (Cell 3) | X | -- |
High Flow (Cell 4) | -- | -- |
Long Term F T average less than -25% | ||
Short Term F T less than -5% | ||
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% 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, 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 kPa and 85 kPa. |
| • | Throttle angle is steady and less than 90%. |
| • | Vehicle speed is less than 132 km/h (82 mph). |
| • | Engine speed is between 600 and 4000 RPM. |
| • | Baro is greater than 70 kPa. |
| • | Air flow is between 3 gm/s and 150 gm/s. |
Conditions for Setting the DTC
| • | Long term fuel trim is at or near maximum authority of -25%. |
| • | Short term fuel trim is at or near maximum authority of -5%. |
| • | 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 consecuitive trip in which the diagnostic has been run and failed. |
| • | If equipped with traction control, the PCM will command the EBTCM via the serial data circuit to turn OFF traction control, and the EBTCM will illuminate the TRACTION OFF lamp. |
| • | The PCM will store conditions which were present when the DTC set as Freeze Frame and Failure Records data. |
Conditions for Clearing the MIL/DTC
| • | The PCM will turn OFF the MIL 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 the scan tool. |
Diagnostic Aids
Check 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 injector(s) faulty - Refer to Fuel Injector Balance Test . |
| • | Check the EVAP Canister for Fuel Saturation. If the EVAP Canister is full of fuel, check 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 . |
| • | Check for a leaking fuel pressure regulator diaphragm by checking the vacuum line to the fuel pressure regulator for the presence of fuel. |
| • | An intermittent TP sensor output will cause the system to go rich due to a false indication of the engine accelerating. |
| • | Poor connection at the PCM - Inspect harness connectors for the following conditions: |
| - | Backed out terminals. Refer to Connector Repairs . |
| - | Improper mating. Refer to Connector Repairs . |
| - | Broken locks. Refer to Connector Repairs . |
| - | Improperly formed or damaged terminals. Refer to Connector Repairs . |
| - | Poor terminal to wire connection. Refer to Connector Repairs . |
| • | Damaged harness - Inspect the wiring harness for damage. If the harness appears to be OK, observe the affected HO2S 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 fault. Refer to Wiring Repairs . |
| . |
Review the Fail Records vehicle mileage since the diagnostic test last failed. This may help determine how often the condition that caused the DTC to be set occurs.
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 Fail 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 | Value(s) | Yes | No |
|---|---|---|---|---|
1 | Was the Powertrain OBD System Check performed? | -- | ||
Are any DTCs set other than P0172? | -- | Go to Applicable DTCs | ||
3 |
Are the displayed values more negative than the specified value? | -25% (L.T.) -5% (S.T.) | ||
Does the scan tool indicate DTC P0172 failed this ignition? | -- | Go to Diagnostic Aids | ||
5 | Visually and physically inspect for debris in the Air Cleaner Assembly. Inspect the air filter element and replace it if necessary. Refer to Air Cleaner Assembly 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 |
Was a problem found? | -- | ||
8 | Start the engine and note the idle quality. Is a low or unsteady idle being experienced? | -- | ||
9 |
Is the Idle Air Control value less than typical Engine Scan Tool Data List ? | -- | ||
10 |
Refer to Fuel System Cleaning . Was a problem found? | -- | ||
11 |
Was a problem found? | -- | ||
12 |
Did the fuel pressure regulator require replacement? | -- | ||
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 |
Was problem found? | -- | ||
15 |
Was a problem found? | -- | ||
16 |
Was a problem found? | -- | ||
17 |
Did both values change to near the Engine Scan Tool Data List ? | 0% | ||
18 |
Was a problem found? | -- | Go to Diagnostic Aids | |
19 | Replace the TP sensor. Refer to Throttle Position Sensor Replacement . 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 DTC P0172 failed this ignition? | -- | System OK |
