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Tim's Auto Repair
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Category: Chevy
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Experience:  Have owned a repair shop for 25 yrs.
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2001 Chevy s10 2.2 L is reading code P0171. its running too

Resolved Question:

2001 Chevy s10 2.2 L is reading code P0171. its running too lean. Ive replace O2 sensors plugs and wires and its still running crappy. What could it be?
Submitted: 6 years ago.
Category: Chevy
Expert:  Tim's Auto Repair replied 6 years ago.


you need to check the fuel pressure..the fuel pump may be weak..these must have no less then.41psi at all times..please print


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fuel pressure

Testing and Inspection


Diagnostic Chart
Diagnostic Chart
Diagnostic Chart
Diagnostic Chart

Circuit Description
When the ignition switch is turned ON, the Powertrain Control Module (PCM) will turn ON the in-tank fuel pump. The in-tank fuel pump will remain ON as long as the engine is cranking or running and the PCM is receiving reference pulses. If there are no reference pulses, the PCM will turn the in-tank fuel pump OFF 2 seconds after the ignition switch is turned ON or 2 seconds after the engine stops running. The in-tank fuel pump is an electric pump within an integral reservoir. The in-tank fuel pump supplies fuel through an in-line fuel filter to the fuel passage within the lower manifold assembly. The fuel pump is designed to provide fuel at a pressure above the pressure needed by the fuel injectors. A fuel pressure regulator, attached to the lower manifold assembly, keeps the fuel available to the fuel injectors at a regulated pressure. Unused fuel is returned to the fuel tank by a separate fuel return pipe.

Test Description
Number(s) below refer to the step number(s) on the Diagnostic Table.


  1. To relieve the fuel pressure, When the ignition switch is ON and the fuel pump is running, the fuel pressure indicated by the fuel pressure gage should be 284-325 kPa (41-47 psi) . This pressure is controlled by the amount of pressure the spring inside the fuel pressure regulator can provide.
  2. A fuel system that drops more than 5 psi in 10 minutes has a leak in one or more of the following areas:
    • The fuel pump check valve.
    • The fuel pump flex pipe.
    • The valve or valve seat within the fuel pressure regulator.
    • The fuel injector(s).
  1. Fuel pressure that during acceleration, cruise or hard cornering may cause a lean condition. A lean condition can cause a loss of power, surging, or misfire. A lean condition can be diagnosed using a scan tool. If an extremely lean condition occurs, the oxygen sensors will stop toggling. The oxygen sensor output voltage(s) will drop below 500 mV. Also, the fuel injector pulse width will increase.

Important: Make sure the fuel system is not operating in the Fuel Cut-Off Mode. This can cause false indications by the scan tool.

  1. When the engine is at idle, the manifold pressure is low (high vacuum). This low pressure (high vacuum) is applied to the fuel pressure regulator diaphragm. The low pressure (high vacuum) will offset the pressure being applied to the fuel pressure regulator diaphragm by the spring inside fuel pressure regulator. When this happens, the result is lower fuel pressure. The fuel pressure at idle will vary slightly as the barometric pressure changes, but the fuel pressure at idle should always be less than the fuel pressure noted in Step 2 with the Engine OFF
  1. A rich condition may result from the fuel pressure being above 325 kPa (47 psi). A rich condition may cause a DTC P0132 or a DTC P0172 to set. Driveability conditions associated with rich conditions can include hard starting (followed by black smoke) and a strong sulfur smell in the exhaust.
  2. This test determines if the high fuel pressure is due to a restricted fuel return pipe or if the high fuel pressure is due to a faulty fuel pressure regulator.
  1. A lean condition may result from the fuel pressure being below 284 kPa (41 psi). A lean condition may cause a DTC P0131 or a DTC P0171 to set. Driveability conditions associated with lean conditions can include hard starting (when the engine is cold), hesitation, poor driveability, lack of power, surging, and misfiring.
  2. Restricting the fuel return pipe causes the fuel pressure to rise above the regulated fuel pressure. Using a scan tool to pressurize the system, the fuel pressure should rise above 325 kPa (47 psi) as the gage outlet hose is pinched.
  1. Check the spark plug associated with a particular fuel injector for fouling or






Diagnostic Chart
Diagnostic Chart (cont'd)

System Description
In order to provide the best possible combination of driveability, fuel economy, and emission control, a Closed Loop air and fuel metering system is used. While in a Closed Loop, the PCM monitors the oxygen sensor signal voltage. The PCM adjusts fuel delivery based on signal voltage. The long and the short term fuel trim values will indicate a change made to the fuel delivery system. The ideal fuel trim values are around 128 (0%). Whenever the oxygen sensor signal is indicating a lean condition, the PCM will add fuel resulting in fuel trim values above 128 (0 to 100%). Whenever a rich condition is detected, the fuel trim values will read below 128 (0 to -100%), indicating that the PCM is reducing the amount of fuel. Whenever the exhaust emissions reach an excessive level due to a lean condition, a fuel trim DTC is set.

Conditions for Setting the DTC
DTCs P0106, P0107, P0108, P0112, P0113, P0117, P0118, P0121, P0122, P0123, P0125, P0131, P0132, P0133, P0134, P0300, P0301, P0302, P0303, P0304, P0325, P0335, P0341 P0342, P0401, P0404, P0405, P0502, P0503, P0506, P0507, P0601, 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 750 RPM and 3400 RPM.
  • Vehicle speed is below 70 mph (113 km/h).
  • MAP is above 27 kPa.
  • Barometric pressure (BARO) is greater than 73.8 kPa.
  • EVAP purge is not occurring.
  • Fuel Trim Index is above 160 (29%).
  • Above conditions present for over 4 seconds.

Action Taken When the DTC Sets

  • The Malfunction Indicator Lamp (MIL) illuminates if the fault is active within the same conditions for 2 out of 80 ignition cycles.


  • The MIL will illuminates after two consecutive ignition cycles in which the diagnostic runs with the fault active.
  • The PCM records the operating conditions at the time the diagnostic fails. The Freeze Frame and Failure Records buffers stores this information.
  • A history DTC is stored.

Conditions for Clearing the MIL/DTC

  • The MIL will turn off after 3 consecutive ignition cycles in which the diagnostic runs without a fault within the freeze frame conditions that the DTC failed.
  • A history DTC will clear after 40 consecutive warm up cycles without a fault.
  • A scan tool can clear the DTCs.

Diagnostic Aids

  • Check for a 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.
  • Check for a damaged harness. If the harness appears to be OK, observe the O2S 1 display on the scan tool while moving the connectors and wiring harness related to the engine harness. A change in the display will indicate the location of the fault.
  • Check the brake power booster check valve.

Test Description
Number(s) below refer to the step number(s) on the Diagnostic Table.

  1. The Powertrain OBD System Check prompts the technician to complete some basic checks and store the freeze frame and failure records data on the scan tool if applicable. This creates an electronic copy of the data taken when the malfunction occurred. The information is then stored on the scan tool for later reference.
  1. Visually/physically checking items which may cause a lean condition may determine the cause of the DTC being set and save diagnosis time.
  1. A vacuum leak can change the Fuel Trim Index and set DTC P0171. This step checks the intake manifold for vacuum leaks.
  1. Contaminants in fuel, such as alcohol or water, can create a lean condition setting DTC P0171. Checking for these contaminants could identify the malfunction.
  1. If no malfunctions have been found at this point and no additional DTCs were set, refer to Diagnostic Aids for additional checks and information.
Tim's Auto Repair, mechanic
Category: Chevy
Satisfied Customers: 15313
Experience: Have owned a repair shop for 25 yrs.
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