| Table 1: | Fuel Trim Cells |
| Table 2: | DTC P0172 - Fuel Trim System Rich |
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 1 HO2S 1 signals 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 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 on bank 1, the PCM will set DTC P0172. The PCM s maximum authority to control long term fuel trim allows a range between -20 percent and +20 percent. The PCM s maximum authority to control short term fuel trim allows a range between -20 percent and +20 percent. The PCM monitors fuel trim under various operating conditions, such as 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) | -- | -- |
Normal (Cell 2) | X | X |
Accel (Cell 3) | X | X |
High Flow (Cell 4) | X | -- |
Long Term FT average less than -18 percent | ||
Short Term FT greater than -2 percent | ||
The vehicle may have to be operated in all of the above conditions 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
| • | The following types of DTCs are NOT present: |
| - | Throttle position (TP) sensor |
| - | Evaporative emission (EVAP) system |
| - | Engine misfire |
| - | Intake air temperature (IAT) sensor |
| - | Manifold absolute pressure (MAP) sensor |
| - | Fuel injector |
| - | Exhaust gas recirculation (EGR) pintle position |
| - | Engine coolant temperature (ECT) sensor |
| - | Crankshaft position (CKP) sensor |
| - | Mass air flow (MAF) sensor |
| - | Vehicle speed sensor (VSS) |
| - | Oxygen (O2) sensor |
| - | EST |
| - | CAM |
| • | 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 percent. |
| • | 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 -20 percent |
| • | Short term fuel trim is at or near maximum authority of -20 percent. |
| • | All conditions have been met in the Fuel Trim Cells. |
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 the scan tool Clear Info function or by disconnecting the PCM battery feed. |
Diagnostic Aids
Check the following items:
| • | Fuel Pressure--Refer to Fuel System Pressure Test (VIN K) . |
| • | 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 Evaporative Emission 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 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 . |
| • | A poor connection at the PCM |
| • | Backed out terminals |
| • | Improper mating |
| • | Broken locks |
| • | Improperly formed or damaged terminals |
| • | Poor terminal to wire connection--Use a corresponding mating terminal to check for proper terminal tension. |
| • | A mis-routed harness--Inspect the MAF sensor harness to ensure that the sensor is not routed too close to high voltage wires such as spark plug leads. |
| • | Damaged harness--If the harness appears to be OK, observe the display on the scan tool while moving connectors and wiring harnesses related to the sensor. A change in the display will indicate the location of the malfunction. Refer to Wiring Repairs in Wiring Systems. |
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. 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? | -- | |||
3 |
Are the displayed values more negative than the specified value? | -10 percent | ||
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 Air Inlet Screen Replacement . Did you find a problem? | -- | ||
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. Value is displayed in counts. Is Idle Air Control value less than the specified value? | 100 counts | ||
10 |
Refer to Fuel System Cleaning . Did you find a problem? | -- | ||
11 |
Did you find a problem? | -- | ||
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 percent-100 percent | ||
14 |
Did you find a problem? | -- | ||
15 |
Did you find a problem? | -- | ||
16 |
Did you find a problem? | -- | ||
17 |
Did both values change to near the specified value? | 0 percent | ||
18 |
Did you find a problem? | -- | Go to Diagnostic Aids | |
19 | Replace the TP sensor. Refer to Throttle Position Sensor Replacement . Is 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 |
