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troubleshooting po171 (fuel system lean?)
02-24-2007, 10:21 AM
Post: #1
Question troubleshooting po171 (fuel system lean?)
i went to auozone to have them read for my serv. engine light the results are "the PCM has dertermined that during the testing. the fuel system for bank  was too lean. (bank 1 identifies the location of cylinder #1, while bank 2 identifies the cylinders on the opposite bank)"
so what does that mean?  
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02-24-2007, 05:22 PM
Post: #2
RE: troubleshooting po171 (fuel system lean?)
Lean codes can be set by something as simple as a vacuum leak. Is this a V-6? Sounds like it if you are referring to B1 vs. B2. I will check for TSB's and do some research.

Cary Joe York
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02-25-2007, 02:04 AM
Post: #3
RE: troubleshooting po171 (fuel system lean?)
its a oldsmobile alero i dont know why its in saab but anyways its a 4cl
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02-25-2007, 10:05 PM
Post: #4
RE: troubleshooting po171 (fuel system lean?)
Since it is a 4 cylinder, there is only one bank. That is bank 1. That is why the code came up for lean, bank 1. You need to check for vacuum leaks, weak fuel pressure, and possibly a MAF that is reading the barometer wrong. Good luck.

Being way2old is why I need help from younger minds.



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02-26-2007, 02:40 PM
Post: #5
RE: troubleshooting po171 (fuel system lean?)
System Description
A Closed Loop air/fuel metering system is used to provide the best possible combination of driveability, fuel economy, and emission control. While in a Closed Loop operation, the PCM monitors the oxygen sensor signal voltage and adjusts the fuel delivery based on the signal voltage. The long term fuel trim values and the short term fuel trim values can indicate any changes to the fuel delivery. Monitor the scan tool for these changes. The ideal fuel trim values are around 128 (0%). If the oxygen sensor signal is indicating a lean condition, the PCM will add fuel resulting in fuel trim values above 128 (0% to 100%). If you detect a rich condition, the fuel trim values will be below 128 (0% to -100%), indicating that the PCM is reducing the amount of fuel delivered. If the exhaust emissions reach an excessive level due to a lean or a rich condition, a fuel trim DTC is set.

Conditions for Setting the DTC (With EGR)


DTCs P0106, P0107, P0108, P0112, P0113, P0117, P0118, P0121, P0122, P0123, P0125, P0131, P0132, P0133, P0134, P0200, P0300, P0301, P0302, P0303, P0304, P0325, P0335, P0341, P0342, P0401, P0404, P0405, P0502, P0503, P0506, P0507, P0601, P0602, P1404, P1133, and P1441, not set.
Intake Air Temperature (IAT) is between -25°C (-13°F) and 115°C (175°F).
Engine Coolant temperature is between 60°C (140°F) and 115°C (239°F).
Engine speed is between 550 RPM and 3400 RPM.
Vehicle speed is below 116km/h (72 mph).
MAP is above 26 kPa.
BARO is greater than 72 kPa.
EVAP purge is not occurring.
Conditions for Running the DTC (Without EGR)


DTCs P0105, P0107, P0108, P0112, P0113, P0117, P0118, P0122, P0123, P0125, P0131, P0132, P0133, P0134, P0200, P0300, P0301, P0302, P0303, P0304, P0325, P0335, P0341, P0342, P0446, P0502, P0503, P0506, P0507, P0601, P0602, P1404, P1133, and P1441, not set.
Intake Air Temperature (IAT) is between -25°C (-13°F) and 115°C (175°F).
Engine Coolant temperature is between 60°C (140°F) and 115°C (239°F).
Engine speed is between 550 RPM and 3400 RPM.
Vehicle speed is below 82 mph (132 km/h).
MAP is above 28 kPa.
BARO is greater than 72 kPa.
EVAP purge is not occurring.
Conditions for Setting the DTC


Fuel Trim Index is above 165 (32%).
Above conditions present for over 4 seconds.
Action Taken When the DTC Sets


The Malfunction Indicator Lamp (MIL) will illuminate if the fault is active within the same conditions for two out of eighty ignition cycles. Or
The MIL will illuminate after two consecutive ignition cycles in which the diagnostic runs with the fault active.
The PCM will record the operating conditions at the time that the diagnostic fails. This information will store in the Freeze Frame and Failure Records buffers.
A history DTC will store.
The coolant fan turns ON.
Conditions for Clearing the MIL/DTC


The MIL turns OFF after three consecutive ignition cycles in which the diagnostic runs without a fault within the freeze frame conditions that the DTC failed.
A history DTC clears after 40 consecutive warm up cycles without a fault.
Clear the MIL/DTC(s) with a scan tool.
Diagnostic Aids

Important: After repairs, use the scan tool Fuel Trim Reset function to reset the long term fuel trim to 128 (0 percent).



The fuel pressure-The system will be lean if the fuel pressure is too low. It may be necessary to monitor the fuel pressure while driving the vehicle at various road speeds and/or loads in order to confirm the fuel pressure. Refer to Fuel System Diagnosis (With EGR) or Fuel System Diagnosis (Without EGR). See: Component Tests\Fuel System Diagnosis
The MAP sensor-An output that causes the PCM to sense a lower than normal manifold pressure (high vacuum) can cause the system to go lean. Disconnecting the MAP sensor will allow the PCM to substitute a fixed (default) value for the MAP sensor. If the lean condition is no longer present when you disconnect the sensor, substitute a known good sensor and recheck the condition. Refer to MAP Sensor Output Diagnosis. See: Component Tests\Manifold Absolute Pressure (MAP) Sensor Diagnosis
Fuel contamination-Water, in even small amounts, near the in-tank fuel pump inlet, can travel to the fuel injector. The water can cause the following conditions:
Plugged fuel filter
Lean exhaust
DTC P0171
Check for a poor O2S 1 or a poor MAP sensor electrical connection at the PCM. Inspect the harness connectors for the following conditions:
Backed out terminals
Improper mating
Broken locks
Improperly formed electrical connectors
Damaged terminals
Poor terminal to wire connection
Check for a damaged harness. Inspect the wiring harness for damage. If the harness appears to be OK, observe the O2S 1 display on the scan tool while moving the electrical connectors and the wiring harness related to the engine harness. A change in the display will indicate the location of the malfunction.
Check the brake power booster check valve for any possible leaks.
Test Description
The number(s) below refer(s) to the step number(s) on the Diagnostic Table.


The Diagnostic System Check prompts you to complete some of the basic checks and to store the freeze frame and failure records data on the scan tool if applicable. This creates an electronic copy of the data captured when the malfunction occurred. The information then stores on the scan tool for later reference.
This step indicates that visually/physically checking any items which can cause a lean condition can determine the cause of the DTC being set and save diagnosis time.
A vacuum leak can change the Fuel Trim Index and set DTC P0171. This step checks the intake manifold for vacuum leaks.
Contaminants in fuel, such as alcohol or water, can create a lean condition setting DTC P0171. Checking for these contaminants could identify the malfunction.
If no malfunction has been present at this point and no additional DTCs were set, refer to Diagnostic Aids above for any additional checks and for information.


Attached File(s)
.gif  P0171 pt.1.gif (Size: 41.34 KB / Downloads: 6)
.gif  P0171 pt.2.gif (Size: 38.8 KB / Downloads: 9)
.gif  P0171 pt.3.gif (Size: 43.88 KB / Downloads: 3)

Cary Joe York
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