For 1990-2009 cars

### Description

The DTC P0174 Fuel Trim System Lean Bank 2 diagnostic monitors the operation of the air/fuel ratio feedback controls. In order to provide the best possible combination of driveability, fuel economy, and emission control, the powertrain control module (PCM) uses a Closed Loop air/fuel metering system. The PCM monitors the heated oxygen sensor (HO2S) signal voltage and when in Closed Loop adjusts fuel delivery based on the HO2S signal voltage. Changes in fuel delivery will be indicated by the long term and the short term fuel trim values that are displayed on the scan tool. The ideal fuel trim value is around 0 percent. The PCM will add fuel when the heated oxygen sensor signal is indicating a lean condition. Additional fuel is indicated by fuel trim values that are above 0 percent. The PCM will reduce the amount of fuel delivered when a rich condition is indicated by the HO2S. Fuel trim values below 0 percent indicate a reduction in fuel. A fuel trim DTC can be set when exhaust emissions reach excessive levels because of a lean or rich condition.

HO2S Indication | Short Term FT Action | Long Term FT Action | Short Term FT Response
--- | --- | --- | ---
Lean Condition | Quick Increase of 1% or More | Slow Increase of 1% or More | Return to 0%
Desired Condition | Stays Near 0% | Stays at Learned +/- Value | Stays Near 0%
Rich Condition | Quick Decrease of -1% or Less | Slow Decrease of -1% or Less | Return to 0%

### Conditions for Running the DTC

• | The barometric pressure (BARO) is more than 75 kPa.

• | The engine coolant temperature (ECT) was between -8°C and +95°C (18°F - 203°F) at engine start up.

• | The intake air temperature (IAT) is between -8°C and +60°C (18°F - 140°F) at engine start up.

### Conditions for Setting the DTC

Any of the following conditions are detected while the ECT is less than 110°C (230°F):

• | The short term fuel trim of bank 2 is more than or equal to 15 percent in 128 firing events.

• | The long term fuel trim of bank 2 is more than or equal to 20 percent in 128 firing events.

• | The total fuel trim of bank 2 is more than or equal to 33 percent in 128 firing events.

### Action Taken When the DTC Sets

• | The PCM illuminates the malfunction indicator lamp (MIL) after 2 consecutive ignition cycles in which the diagnostic runs with the fault active.

• | The PCM records the operating conditions at the time the diagnostic fails. This information is stored in the Freeze Frame buffer.

### Conditions for Clearing the MIL/DTC

• | The MIL turns OFF after 3 consecutively passing cycles without a fault present.

• | A history DTC will clear after 40 consecutive warm-up cycles without a fault.

• | Perform the scan tool Clear DTC Information function in order to clear the DTC.

### Diagnostic Aids

Check for any of the following conditions:

• | Check for vacuum leaks.

• | Check for exhaust leaks. Refer to Exhaust Leakage in Engine Exhaust.

• | Inspect the HO2S bank 2 sensor 1 for a short to ground in the signal circuit. Monitor the HO2S signal voltage on a scan tool. If the displayed voltage is near 0 volts, check for a physical short to ground or water intrusion into the wiring harness and the sensor housing. Water can create a short to ground in the HO2S signal circuit causing a false lean indication.

• | Check for a contaminated oxygen sensor--Silicone and other contaminants can cause the oxygen sensor to provide false lean exhaust readings. If contamination is found, determine the cause and correct the condition before replacing the HO2S.

• | Check for a damaged wiring harness--Inspect the wiring harness for damage. If the harness appears to be OK, observe the heated oxygen sensor (HO2S) bank 1 sensor 1 display on the scan tool while moving the connectors and the wiring harnesses related to the sensor. A change in the display will indicate the location of the fault.

• | Check for fuel contamination--Small amounts of water can be delivered to the fuel injectors and cause a lean exhaust indication. A lean exhaust indication can also be caused by too much alcohol in the fuel. Refer to Alcohol/Contaminants-in-Fuel Diagnosis .

• | A malfunctioning mass air flow (MAF) sensor--Refer to DTC P0101 .

• | Check for the correct fuel pressure--The fuel system will be lean if the fuel pressure is too low. In order to determine the cause of DTC P0174 monitoring the fuel pressure while driving the vehicle at various road speeds may be necessary. Refer to Fuel System Diagnosis .

If a DTC P0174 is intermittent, driving the vehicle under the following conditions can verify whether the fault is present. Perform the scan tool Clear DTC Information function. Road test the vehicle while monitoring the DTC P0174 diagnostic on the scan tool under the Not Ran Since Code Cleared selection in the DTC Information menu. If a DTC P0174 appears in the Not Ran Since Code Cleared list, the DTC P0174 diagnostic has not yet run. When the DTC P0174 does not appear in the Not Ran Since Code Cleared list, the DTC P0174 diagnostic has run. If the MIL is NOT illuminated, and there is no Pending DTC Status in DTC Information, the DTC P0174 diagnostic has passed. The DTCs MUST be cleared in order to view the Current Status of the Not Ran Since Code Cleared list. DO NOT forget that the Not Ran Since Code Cleared list only indicates that the test has run, not whether the test passed or failed. The DTC Information screen must be checked for Current or Pending status, in order to determine the outcome of the diagnostic test involved.

If a DTC P0174 cannot be duplicated, the information included in the Freeze Frame data can be useful in determining vehicle operating conditions when the DTC was first set.

### Test Description

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

1. The Diagnostic System Check-Engine Controls prompts the technician to complete some basic checks and store the Freeze Frame data on the scan tool if applicable. This creates an electronic copy of the data taken when the fault occurred. The information is then stored in the scan tool for later reference.

2. Total Fuel Trim is the addition of Short Term Fuel Trim (FT) and Long Term Fuel Trim (FT) numbers. For example, if Short Term FT is 11 percent, and Long Term FT is 15 percent, then Total Fuel Trim is 26 percent.

3. Engine control sensors that are found to be out range or fixed, can cause the engine to run lean. Refer to the applicable sensor DTC table for direction in diagnosing any possible sensor error.

4. This step checks for inadequate fuel delivery from a fuel injector. The fault may be a fuel injector or the fuel injector electrical circuit.

5. Clearing the DTCs by removing the DOME fuse also clears the PCM long term memory. Using the scan tool to clear DTCs leaves the long term memory active. DTC emitting diagnostics may not run to completion as expected unless the long term memory is cleared. Removal of the DOME fuse also clears the time display of the radio. The DTC P0174 diagnostic can be monitored on the scan tool under the Not Ran Since Code Cleared selection in the DTC Information menu. If a DTC P0174 appears in the Not Ran Since Code Cleared list, the DTC P0174 diagnostic has not yet run. When the DTC P0174 does not appear in the Not Ran Since Code Cleared list, the DTC P0174 diagnostic has run. If the MIL is NOT illuminated, and there is no Pending DTC Status in DTC Information, the DTC P0174 diagnostic has passed. The DTCs MUST be cleared in order to view the Current Status of the Not Ran Since Code Cleared list. DO NOT forget that the Not Ran Since Code Cleared list only indicates that the test has run, not whether the test passed or failed. The DTC Information screen must be checked for Current or Pending status, in order to determine the outcome of the diagnostic test involved.

Step | Action | Value(s) | Yes | No
--- | --- | --- | --- | ---
1 | Did you perform the Diagnostic System Check-Engine Controls? | -- | Go to Step 2 | Go to Diagnostic System Check - Engine Controls
2 |

1. Start the engine.
2. Allow the engine to reach a normal operating temperature.
3. Operate the vehicle until the scan tool indicates Closed Loop.
4. Observe the Short Term FT parameter on the scan tool.

Is the scan tool display less than the specified value?

15 %

Go to Step 3

Go to Step 4

3 |

1. Turn ON the ignition, leaving the engine OFF.
2. Perform the scan tool Clear DTC Information function.
3. Operate the vehicle within the Freeze Frame conditions as noted.
4. Observe the Total Fuel Trim parameter on the scan tool.

Was the scan tool display less than the specified value while operating the vehicle within the Freeze Frame conditions?

33 %

Go to Diagnostic Aids

Go to Step 4

4 |

1. Visually and physically check for any of the following conditions:

• | Any splits, kinks, and improper connections in the vacuum hoses

• | The correct operation positive crankcase ventilation (PCV) valve

• | Any exhaust system leaks near the HO2S

• | The correct operation of the evaporative emission (EVAP) control system

2. Repair as necessary.

Did any of the above checks isolate a condition requiring a repair?

--

Go to Step 11

Go to Step 5

5 |

1. Start the engine.
2. Run the engine to normal operating temperatures.
3. Observe the operation of the following sensors at idle, with a scan tool:

• | The manifold absolute pressure (MAP) sensor

• | The engine coolant temperature (ECT) sensor

• | The throttle position (TP) sensor

4. Compare the observed sensor readings with the Typical Scan Tool Data values located in the engine scan tool data list. Refer to Scan Tool Data List .
5. Inspect the vehicle for the cause of any errant sensor readings and repair as necessary.

Was a repair necessary?

--

Go to Step 11

Go to Step 6

6 |

1. Turn OFF the ignition.
2. Install a J 34730-1A Fuel Pressure Gage or an equivalent. Refer to Fuel Pressure Gage Installation and Removal .
3. Start the engine.
4. Observe the fuel pressure with the engine idling at normal operating temperatures.

Is the fuel pressure within the specified range?

270-310 kPa (39-5 psi)

Go to Step 7

Go to Fuel System Diagnosis

7 | Check the fuel for excessive water, alcohol, or other contaminants. Refer to Alcohol/Contaminants-in-Fuel Diagnosis . Was the fuel contaminated? | -- | Go to Step 8 | Go to Step 9
8 | Replace any contaminated fuel. Refer to Fuel System Cleaning . Is the action complete? | -- | Go to Step 11 | --
9 | Check the fuel injectors for any of the following conditions:

• | The discharge of fuel is regular and even. Refer to Fuel Injector Balance Test with Special Tool .

• | The fuel is dispersed in a conical spray pattern.

• | The electrical resistance of the injector coils are all within specifications. Refer to Fuel Injector Solenoid Coil Test - Engine Coolant Temperature Between 10-35 Degrees C (50-95 Degrees F) .

Did the fuel injectors pass all the checks?

--

Go to Diagnostic Aids

Go to Step 10

10 | Replace any faulty fuel injectors. Refer to Fuel Injector Replacement . Is the action complete? | -- | Go to Step 11 | --
11 |

1. Turn OFF the ignition.
2. Remove the DOME fuse for 30 seconds.
3. Install the DOME fuse.
4. Reset the radio clock time. Refer to Radio/Audio System Description and Operation in Entertainment.
5. Start the engine.
6. Operate the vehicle within the Conditions for Running the DTC as specified in the supporting text or until the DTC P0174 diagnostic test has run.

Does the DTC run and pass?

--

Go to Step 12

Go to Step 2

12 | With a scan tool, observe the stored information, Capture Info. Does the scan tool display any DTCs that you have not diagnosed? | -- | Go to Diagnostic Trouble Code (DTC) List | System OK