For 1990-2009 cars

### Circuit Description

While in Closed Loop, the engine control module (ECM) monitors the heated oxygen sensor (HO2S) 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 diagnostic trouble code (DTC). The Long Term Fuel Trim diagnostic parameter is an average of several of the long term speed load learn cells which the ECM selects based on the engine speed and the engine 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, 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 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 failure was during a non-typical driving condition, the MIL may remain ON more 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 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 diagnostic or any other emission related diagnostic.

• | The scan tool clears the MIL/DTC.

### Diagnostic Aids

• | Use the J 35616 connector test adapter kit for any test that requires probing the ECM harness connector or a component harness connector. Using this kit prevents damage to the harness or component terminals. Refer to Using Connector Test Adapters in Wiring Systems.

• | If a DTC P0174 cannot be duplicated, the DTC may have been caused by the customer running the vehicle out of fuel.

• | If DTC P0171 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 also 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 .

• | If a repair is necessary refer to Wiring Repairs or Connector Repairs in Wiring Systems.

### Test Description

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

1. This step determines if the fault is present.

2. This process may help in order to recreate the malfunction.

3. This step determines if both banks are running lean.

4. This step addresses areas of concern that could cause both banks of the engine to run lean.

#### DTC P0174 - Fuel Trim System Lean Bank 2

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 Long Term Fuel Trim for Bank 1 with the scan tool.

Is the LT FT less than the specified value?

+23% LT

Go to Step 4

Go to Step 5

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 30 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 P0171 also set? | -- | Go to Step 6 | Go to Step 7
6 | Visually/physically inspect the following items:

• | Vacuum hoses for splits, kinks, and proper connections. Refer to Emission Hose Routing Diagram for underhood vacuum hose routing diagrams.

• | Air induction system after the MAF sensor for leaks.

• | Throttle body and intake manifold for leaks.

• | Crankcase ventilation system for leaks. Refer to Crankcase Ventilation System Diagnosis .

• | Test for contaminated fuel. Refer to Alcohol/Contaminants-in-Fuel Diagnosis .

• | ECM and the sensor grounds are clean, tight, and in the proper locations. Refer to Ground Distribution Schematics in Wiring Systems.

• | Engine mechanical failure. Refer to Engine Noise Diagnosis in Engine Mechanical.

Did you find and correct the condition?

--

Go to Step 9

Go to Fuel System Diagnosis

7 | Visually/physically inspect the following items:

• | Bank 2 for exhaust leaks and missing or loose exhaust hardware.

• | Bank 2 HO2S 1 is installed securely and the electrical connector is not contacting the exhaust system.

• | Engine mechanical failure. Refer to Engine Noise Diagnosis in Engine Mechanical.

• | Vacuum leaks that only affect Bank 2 (intake manifold, upper plenum O-rings, injector O-rings, etc.).

Did you find and correct the condition?

--

Go to Step 9

Go to Step 8

8 | Test for a malfunctioning oxygen sensor or circuit. Go to DTC P0152 HO2S Circuit High Voltage Bank 2 Sensor 1 . Did you find and correct the condition? | -- | 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