| 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 powertrain control module (PCM) monitors the Bank 1 HO2S 1 and Bank 2 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 PCMs maximum authority to control Long Term Fuel Trim allows a range between -20 percent and +20 percent. The PCMs 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, 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 | -- |
L.T. Fuel Trim average less than -18 percent | ||
S.T. Fuel Trim more than -2 percent | ||
Conditions for Running the DTC
| • | No VSS, EST, CKP, CAM, TP, Misfire, Fuel Injector, MAF, O2, MAP, EGR, EVAP, ECT, or IAT DTCs are set. |
| • | Engine coolant temperature between is 20°C-110°C (68°F-230°F). |
| • | Intake air temperature is between -18°C-70°C (0°F-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 3-150 gm/s. |
| • | Bank 1 Long Term Fuel Trim is at or near maximum authority of -20 percent. |
| • | Bank 1 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 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 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:
| • | 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 the 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 the 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 0-100 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 |
| - | 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 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.
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 Failure Records conditions are being duplicated, the rich condition is intermittent. Go to Diagnostic Aids for additional information on diagnosing intermittent conditions.
Step | Action | Values | 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% | ||
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 |
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 Idle Air Control display on the scan tool. Is the Idle Air Control value less than the specified value? | 100 | ||
10 |
Did you find and correct the condition? | -- | ||
11 |
Did you find and correct the condition? | -- | ||
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 |
Did you find and correct the condition? | -- | ||
16 |
Did you find and correct the condition? | -- | ||
17 |
Did you find and correct the condition? | -- | ||
18 |
Did you find and correct the condition? | -- | Go 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 DTC P0172 failed this ignition? | -- | System OK |
