For 1990-2009 cars

### Circuit Description

While in Closed Loop, the ECM monitors the heated oxygen sensor signal voltage and adjusts the fuel delivery based on the signal voltage. A change made to fuel delivery will be indicated by the Long and 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. The 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 fuel trim diagnostic parameter is a value calculated on the basis of speed and load.

### Conditions for Running the DTC

• | Closed loop operation

• | Fuel trim enabled

### Conditions for Setting the DTC

The ECM detects fuel trim values more than +23 percent for long term fuel trim or more than +25 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.

• | On the first drive trip that this diagnostic fails (Fail Pending), the information will be stored in the Freeze Frame. Any data that was previously 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 fault 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 turns off the MIL after three consecutive drive trips that the diagnostic runs and passes within the same conditions that the DTC last set.

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

• | The ECM battery voltage is interrupted.

• | The scan tool clears the MIL/DTC.

### Diagnostic Aids

• | If a DTC P0171 cannot be duplicated, the DTC may have been caused by the customer running the vehicle out of fuel.

• | If DTC P0174 is also set, this indicates that both banks of the engine are operating lean. Inspect all areas of the engine for a vacuum leak. Also inspect the crankcase ventilation (PCV) system for leaks. Make sure that the engine oil fill cap is tight. Check that the engine oil dip stick is fully seated. A skewed mass air flow (MAF) sensor may be the cause of these DTCs.

• | Fuel delivery--A fuel delivery malfunction can cause this DTC to set. Thoroughly inspect all items that cause a lean condition.

• | Fuel contamination--Water, even in small amounts, near the in-tank fuel pump inlet, can be delivered to the injectors. The water causes a lean condition and can set a DTC P0171.

• | Secondary AIR System--Inspect the AIR system for proper operation and verify that all the components are connected.

• | Intermittents--For an intermittent, refer to Symptoms .

### Test Description

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

1. This step determines whether the 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 with in 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 P0174 is also set, this indicates that both banks of the engine are operating lean. Inspect all items that would cause both banks to operate lean.

4. Inspect all areas of the engine for a vacuum leak. Also inspect thepositive crankcase ventilation (PCV) system for leaks. Make sure that the engine oil fill cap is tight. Check that the engine oil dip stick is fully seated.

#### DTC P0171 - Fuel Trim System Lean 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 | Is DTC P1460 set? | -- | Go to DTC P1460 Misfire Detected With Low Fuel Level | Go to Step 3
3 | Important: If any DTCs are set (except P0171 and P0174), refer to those DTCs before proceeding with this diagnostic.

1. Connect a scan tool.
2. Allow the engine to run at a normal operating temperature.
3. Verify that the fuel system is in Closed Loop.
4. Monitor the Short Term and the Long Term Fuel Trim for Bank 1 with the scan tool.

Is either parameters more than the specified value?

+8.5% LT

+23% ST

Go to Step 5

Go to Step 4

4 |

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 that are recorded in the Freeze Frame as possible. Refer to Conditions for Running the DTC.
7. Check for any DTCs.

Is this DTC set?

--

Go to Step 5

Go to Diagnostic Aids

5 | Is DTC P0174 also set? | -- | Go to Step 6 | Go to Step 7
6 | Perform the following visual/physical inspections and tests:

• | Inspect the vacuum hoses for splits, kinks, and proper connections.

• | Inspect the throttle body and the intake manifold for any vacuum leaks.

• | Inspect the crankcase ventilation valve and/or system for leaks.

• | Test for contaminated fuel. Refer to Crankcase Ventilation System Diagnosis .

• | Verify that the ECM and the sensor grounds are clean, tight, and in the proper locations.

• | Test the air induction system after the MAF sensor for air leaks.

• | Inspect/test for engine mechanical failure. Refer to Engine Mechanical.

Did you find and correct a problem?

--

Go to Step 9

Go to Fuel System Diagnosis

7 | Perform the following visual/physical inspections and tests:

• | Inspect Bank 1 for exhaust leaks and missing or loose exhaust hardware.

• | Verify that the Bank 1 HO2S 1 is installed securely and the electrical connector is not contacting the exhaust system or the ignition wires.

• | Inspect/test for any engine mechanical failure. Refer to Engine Mechanical.

• | Inspect/test for vacuum leaks that only affect Bank 1 (the intake manifold, the injector O-rings, etc.).

Did you find and correct a problem?

--

Go to Step 9

Go to Step 8

8 | Test for a malfunctioning oxygen sensor or circuit. Go to DTC P0132 HO2S Circuit High Voltage Bank 1 Sensor 1 . Did you find and correct a problem? | -- | Go to Step 9 | Go to Fuel System Diagnosis
9 |

1. Using a 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 that the diagnostic Ran.

Does the scan tool indicate that the diagnostic Passed?

--

Go to Step 10

Go to Step 2

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