For 1990-2009 cars

### 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 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%; if the HO2S signals are indicating a lean condition the PCM will add fuel, resulting in fuel trim values above 0%. If a rich condition is detected, the fuel trim values will be below 0%, indicating that the PCM is reducing the amount of fuel delivered. If an excessively lean condition is detected on bank 1, the PCM will set DTC P0171. The PCM s maximum authority to control long term fuel trim allows a range between -23% and +16%. The PCM s maximum authority to control short term fuel trim allows a range between -11% and +20%. 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 weighted (assigned a percentage of importance in determining the total fuel trim status for the diagnostic) as follows:

Manaul Transaxle

• | Idle, purge commanded (Cell 10 - approximately 27%)

• | Normal driving, purge commanded (Cell 2 - approxmately 18%)

• | Acceleration, purge commanded (Cell 3 - approximately 9%)

• | High Flow, purge commanded (Cell 4 - approximately 9%)

• | Normal driving, no purge commanded (Cell 6 - approximately 27%)

• | Acceleration, no purge commanded (Cell 7 - approximately 9%)

Automatic Transaxle

• | Idle, purge commanded (Cell 10 - approximately 15%)

• | Normal driving, purge commanded (Cell 2 - approxmately 25%)

• | Deceleration, no purge commanded (Cell 5 - approximately 5%)

• | Normal driving, no purge commanded (Cell 6 - approximately 40%)

• | Idle, no purge commanded (Cell 9 - approximately 5%)

### Conditions for Setting the DTC

• | No TP sensor, EVAP system, HO2S, misfire, IAT sensor, MAP sensor, Idle speed, fuel injector(s), IC/Bypass, EGR Pintle, EGR Flow, ECT sensor, CKP sensor, CMP sensor, or MAF sensor DTCs are set.

• | Engine coolant temperature between 20°C (68°F) and 110°C (230°F).

• | Intake air temperature is between -18°C (0°F) and 70°C (158°F).

• | Manifold absolute pressure is between 15 kPa and 85 kPa.

• | Throttle angle is steady and less than 90%.

• | Vehicle speed is less than 132 km/h (82 mph).

• | Engine speed is between 600 and 4000 RPM.

• | Baro is greater than 70 kPa.

• | Air flow is between 3 gm/s and 150 gm/s.

• | Bank 1 long term fuel trim is at or near maximum authority of +16%.

• | Bank 1 short term fuel trim is at or near maximum authority of +20%.

• | All of the conditions described above have been met in the Fuel Trim Cells.

### Action Taken When the DTC Sets

• | The PCM will illuminate the 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 Fail Records data.

### Conditions for Clearing the MIL/DTC

• | The PCM will turn the MIL OFF 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 Clear Information function or by disconnecting the PCM battery feed.

### Diagnostic Aids

Check for the following conditions:

• | Heated oxygen sensor wiring - The sensor pigtail may be routed incorrectly and contacting the exhaust system.

• | Poor PCM to engine block grounds

• | Poor connection at the PCM - Inspect harness connectors for the following conditions:

- | Backed out terminals.

- | Improper mating.

- | Broken locks.

- | Improperly formed or damaged terminals.

- | Poor terminal to wire connection.

• | Fuel pressure low - The system will go lean if pressure is too low. The PCM can compensate for some decrease. However, if fuel pressure is too low, a DTC P0171 may be set. Refer to Fuel System Pressure Test .

• | Fuel injector(s) faulty - Refer to Fuel Injector Balance Test .

• | Vacuum leaks - Check for disconnected or damaged vacuum hoses and for vacuum leaks at the intake manifold, throttle body, EGR system, and crankcase ventilation system.

• | Exhaust leaks - An exhaust leak may cause outside air to be pulled into the exhaust gas stream past the HO2S, causing the system to appear lean. Check for exhaust leaks that may cause a false lean condition to be indicated.

• | Disconnect the MAF sensor and see if the lean condition is corrected. If so, replace MAF sensor. Refer to Mass Airflow Sensor Replacement

• | Fuel contamination - Water, even in small amounts, can be delivered to the fuel injectors. The water can cause a lean exhaust to be indicated. Excessive alcohol in the fuel can also cause this condition. Refer to Alcohol/Contaminants-in-Fuel Diagnosis .

• | Poor connection at PCM - Inspect harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and poor terminal to wire connection(s).

• | Damaged harness - Inspect the wiring harness for damage. If the harness appears to be OK, observe the affected HO2S 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 fault.

Review the Fail Records vehicle mileage since the diagnostic test last failed. This may help determine how often the condition that caused the DTC to be set occurs.

### Test Description

Number(s) below refer to the step number(s) on the Diagnostic Table:

1. DTCs other than P0171 and P0174 may indicate a condition present which may cause a lean condition. If this is the case, repairing the condition which caused the other DTC will most likely correct the DTC P0171/P0174.

2. If the DTC P0171 test passes while the Fail Records conditions are being duplicated, the lean condition is intermittent. Refer to Diagnostic Aids.

#### DTC P0171 - Fuel Trim System Lean Bank 1

Step | Action | Value(s) | Yes | No
--- | --- | --- | --- | ---
1 | Was the Powertrain On-Board Diagnostic (OBD) System Check performed? | -- | Go to Step 2 | Go to the Powertrain On Board Diagnostic (OBD) System Check
2 | Are any DTCs set other than P0171 and P0174? | -- | Repair the other DTCs first | Go to Step 3
3 |

1. Start and run vehicle in closed loop.
2. Observe BANK 1 L.T. FUEL TRIM and BANK 1 S.T. FUEL TRIM display on the scan tool.

Are the displayed values near or above the specified values?

• | L.T. FUEL TRIM is the first specified value.

• | S.T. FUEL TRIM is the second specified value.

+16%

+20%

Go to Step 5

Go to Step 4

4 |

1. Review and record the scan tool Fail Records data.
2. Clear the DTC P0171/P0174 and operate the vehicle to duplicate the Fail Records conditions.
3. Monitor the scan tool Specific DTC info for DTC P0171 while operating the vehicle to duplicate the Fail Records conditions.
4. Continue operating the vehicle until the DTC P0171 test runs and note test result.

Does the scan tool indicate DTC P0171 failed this ign?

--

Go to Step 5

Go to Diagnostic Aids

5 | Was DTC P0174 also set? | -- | Go to Step 6 | Go to Step 17
6 | Visually and physically inspect the vacuum hoses for splits, kinks, and improper connections and repair any problem found. Did your inspection reveal a problem requiring repair? | -- | Go to Step 23 | Go to Step 7
7 | Visually and physically inspect the PCV Valve for proper installation and repair any problem found. Refer to Crankcase Ventilation System Inspection . Did your inspection reveal a problem requiring repair? | -- | Go to Step 23 | Go to Step 8
8 |

1. Inspect the throttle body inlet screen for damage or for the presence of foreign objects which may partially block the airflow sample through the MAF sensor.
2. Correct any problem that is found as necessary.

Did your inspection of the throttle body reveal a condition requiring repair?

--

Go to Step 23

Go to Step 9

9 | Start the engine and note the idle quality. Is a high or unsteady idle being experienced? | -- | Go to Step 10 | Go to Step 13
10 | With the engine idling, observe the Idle Air Control display on the scan tool. Is the displayed value above the specified value? Value displayed is in Counts. | 5 | Go to Step 12 | Go to Step 11
11 |

1. Visually and physically inspect the throttle body, intake manifold, EGR valve and the EGR feed pipes for vacuum leaks.
2. Repair any vacuum leaks as necessary.

Did your inspection reveal a vacuum leak?

--

Go to Step 23

Go to Step 12

12 | Perform the Idle Air Control (IAC) Valve Check. Refer to Idle Air Control (IAC) System Diagnosis Did the Idle Air Control (IAC) Valve Check isolate a problem requiring repair? | -- | Go to Step 23 | Go to Step 13
13 | Check the fuel for excessive water, alcohol, or other contaminants. Refer to Alcohol/Contaminants-in-Fuel Diagnosis . Refer to Engine Fuel. Was the fuel contaminated? | -- | Go to Step 23 | Go to Step 14
14 |

1. Visually and physically inspect the PCM injector grounds, power grounds and sensor grounds to ensure that they are clean, tight, and in their proper locations.
2. If a faulty ground condition is present, correct it as necessary. Refer to Wiring Repairs .

Did your inspection reveal a condition requiring repair?

--

Go to Step 23

Go to Step 15

15 |

1. Disconnect the MAF sensor electrical connector.
2. Operate the vehicle in Closed Loop while monitoring the Bank 1 S.T. Fuel Trim display on the scan tool

Does Bank 1 S.T. Fuel Trim value decrease to near the specified value?

0%

Go to Step 22

Go to Step 16

16 | Perform the Fuel System Pressure Test . Refer to Fuel System Pressure Test . Did the Fuel System Pressure Test isolate a condition requiring repair? | -- | Go to Step 23 | Go to Step 17
17 | Perform the Evaporative Emission System Check. Refer to Evaporative Emission Control System Diagnosis Did the Evaporative Emission System Check isolate a problem requiring repair? | -- | Go to Step 23 | Go to Step 18
18 |

1. Visually and physically inspect the intake manifold, injector O-rings, EGR adapter, EGR valve and the EGR feed pipes for vacuum leaks.
2. Repair any problem that is found

Did your inspection reveal a problem requiring repair?

--

Go to Step 23

Go to Step 19

19 | Visually and physically inspect the Bank1 exhaust manifold for leaks and loose or missing hardware and correct any problem found. Did your inspection reveal a problem requiring repair? | -- | Go to Step 23 | Go to Step 20
20 | Perform the Injector Balance Test . Refer to Fuel Injector Balance Test . Did the Injector Balance Test isolate a problem requiring repair? | -- | Go to Step 23 | Go to Step 21
21 |

1. Visually and physically inspect the Bank 1 HO2S to ensure that it is installed securely and that the Bank 1 HO2S pigtail and wiring harness are not contacting the exhaust or otherwise damaged.
2. If a problem is found, correct it as necessary.

Did your inspection reveal a problem requiring repair?

--

Go to Step 23

Go to Diagnostic Aids

22 | Replace the MAF sensor. Refer to Mass Airflow Sensor Replacement . Is the action complete? | -- | Go to Step 23 | --
23 |

1. Review and record the scan tool Fail Records data.
2. Clear the DTC P0171 and operate the vehicle to duplicate the Fail Records conditions.
3. Monitor the scan tool Specific DTC info for DTC P0171 while operating the vehicle to duplicate the Fail Records conditions.
4. Continue operating the vehicle until the DTC P0171 test runs and note test result.

Does the scan tool indicate DTC P0171 failed this ignition?

--

Go to Step 2

System OK