For 1990-2009 cars

### Circuit Description

In order to provide the best possible combination of driveability, fuel economy and emission control, a Closed Loop air/fuel metering system is used. While in a Closed Loop operation, the ECM monitors the heated oxygen sensor signal voltage and adjusts the fuel delivery based on the signal voltage. A change made to the fuel delivery will be indicated by the Long and the Short Term Fuel Trim values. The Short Term Fuel Trim values change rapidly in response to the HO2S signal voltages. These changes fine tune the engine fueling. The Long Term Fuel Trim values change in response to trends in the Short Term Fuel Trim. The Long Term Fuel Trim makes coarse adjustments to the fueling in order to re-center and restore control to the Short Term Fuel Trim. Short and the Long Term Fuel Trim can be monitored with a scan tool. Ideal fuel trim values are around 0 percent. A positive Fuel Trim value indicates that the ECM is adding fuel in order to compensate for a lean condition. A negative Fuel Trim value indicates that the ECM is reducing the amount of fuel in order to compensate for a rich condition. If an excessively Rich or Lean condition is detected, the ECM will set a DTC. The long term (Additive and Multiplicative) fuel trim diagnostic parameter is a calculated value from several of the long term speed load learn cells which are selected based on speed and load.

### Conditions for Running the DTC

• | Closed loop operation

• | Fuel trim enabled

### Conditions for Setting the DTC

The ECM detects the fuel trim values that are more than -8.5 percent for long term fuel trim or more than -23 percent for short term fuel trim.

### Action Taken When the DTC Sets

• | The ECM will illuminate the malfunction indicator lamp MIL on the second consecutive drive trip that the diagnostic runs and fails, if the diagnostic fails under the same conditions (the load, the RPM, the temperature, etc.) as the previous ignition cycle that the test ran and failed.

• | The first drive trip that this diagnostic fails (Fail Pending), this information will be stored in the Freeze Frame. Any data previously that was stored in the Freeze Frame will be overwritten by the Freeze Frame data for this DTC. The only exception to this is if a Misfire DTC was already recorded in the Freeze Frame. In this case, the Misfire data stays in the Freeze Frame and the Fuel Trim data is not stored.

### Conditions for Clearing the MIL/DTC

| Important: If the last failure was during a non-typical driving condition, the MIL may remain on longer than three drive trips. Review the Freeze Frame for the last failure conditions.

• | The ECM will turn off the MIL after three consecutive drive trips that the diagnostic runs and does not fail within the same conditions that the DTC last failed.

• | A History DTC clears after forty consecutive warm-up cycles if no failures report by this or any other emission related diagnostic.

• | The ECM battery voltage is interrupted.

• | The scan tool clears the MIL/DTC.

### Diagnostic Aids

Refer to Symptoms for the Intermittents.

### Test Description

The numbers below refer to the step numbers on the diagnostic table.

1. A more negative number, e.g.: -9 percent, indicates a fault is present.

2. Using the Freeze Frame data may aid in locating an intermittent condition. If the DTC cannot be duplicated, review the information in the Freeze Frame. Try to operate the vehicle within the same Freeze Frame conditions, the RPM, the MAF, the vehicle speed, the temperature, etc., that were noted. This process may help in order to recreate the malfunction.

3. If DTC P0175 is also set, this indicates both banks of the engine are operating rich. Inspect items that would cause both banks to operate rich.

#### DTC P0172 - Fuel Trim System Rich Bank 1

Step | Action | Values | Yes | No
--- | --- | --- | --- | ---
1 | Did you perform the Powertrain On-Board Diagnostic (OBD) System check? | -- | Go to Step 2 | Go to Powertrain On Board Diagnostic (OBD) System Check
2 | Important: If any DTCs are set (except P0175), refer to those DTCs before proceeding with this diagnostic.

1. Connect the scan tool.
2. Allow the engine to run at a normal operating temperature.
3. Verify that the fuel system is in Closed Loop.
4. Use a scan tool in order to monitor the short term and the long term fuel trim for bank 1.

Is either parameter more negative than the specified values?

-8.5% LT

-23% ST

Go to Step 4

Go to Step 3

3 |

1. Turn ON the ignition with the engine OFF.
2. Review the Freeze Frame data for this DTC and note the parameters.
3. Clear the DTCs.
4. Turn OFF the ignition for 15 seconds.
5. Start the engine.
6. Operate the vehicle within the conditions required for this diagnostic to run, and as close to the conditions recorded in Freeze Frame as possible. Refer to Conditions for Running the DTC.
7. Check for any DTCs.

Is this DTC set?

--

Go to Step 4

Go to Diagnostic Aids

4 | Is DTC P0175 also set? | -- | Go to Step 5 | Go to Step 6
5 | Visually/physically inspect the following items:

• | A collapsed air intake duct

• | The air filter element for being restricted, refer to Air Induction System .

• | The MAF sensor for being installed in the proper direction and for foreign objects blocking the inlet screen, refer to Mass Airflow Sensor/Intake Air Temperature Sensor Replacement .

• | For fuel in the fuel pressure regulator hose, refer to Alcohol/Contaminants-in-Fuel Diagnosis .

| Important: When the fuel system check is finished, return to the beginning of this table.

Did you find and correct the problem?

--

Go to Step 8

Go to Fuel System Diagnosis

6 | Inspect the Bank 1 injectors for leaks. Refer to Fuel Injector Balance Test . Did you find and correct the problem? | -- | Go to Step 8 | Go to Step 7
7 | Test for a malfunctioning oxygen sensor or circuit. Go to DTC P0131 HO2S Circuit Low Voltage Bank 1 Sensor 1 . Did you find and correct a problem? | -- | Go to Step 8 | Go to Fuel System Diagnosis
8 |

1. Using the scan tool, clear the DTCs.
2. Start the engine.
3. Allow the engine to idle until the engine reaches the normal operating temperature.
4. Select DTC and the Specific DTC function.
5. Enter the DTC number that was set.
6. Operate the vehicle within the Conditions for Running this DTC until the scan tool indicates the diagnostic Ran.

Does the scan tool indicate the diagnostic Passed?

--

Go to Step 9

Go to Step 2

9 | Does the scan tool display any additional undiagnosed DTCs? | -- | Go to the applicable DTC table | System OK