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DTC P0171: 1/1 Fuel System Lean: Testing

NOTE: 1/1 HO2S may also be referred to as an upstream oxygen sensor. On models with Federal emissions, 1/1 HO2S is located in exhaust downpipe in front of catalytic converter. On models with Calif. emissions, 1/1 HO2S is located in front exhaust manifold.
  1. Turn ignition on. Using scan tool, read DTCs. If GOOD TRIP counter is displayed for DTC P0171 and displayed count is "0", go to next step. If GOOD TRIP counter is not displayed for DTC P0171 or displayed count is not "0", go to step  16 .
  2. Turn ignition off. Release fuel pressure. See FUEL SYSTEM PRESSURE RELEASE  . Remove test port cap from fuel rail. Install a 0-60 psi (0-4.2 kg/cm2 ) fuel pressure gauge to test port on fuel rail. Turn ignition on. Using scan tool, actuate ASD fuel system test. Allow fuel pressure gauge to stabilize. Stop ASD fuel system activation. Fuel pressure should be 44.2-54.2 psi (3.23-3.96 kg/cm2 ). If fuel pressure is less than specified, go to next step. If fuel pressure is more than specified, replace fuel filter/pressure regulator. Fuel filter/fuel pressure regulator is located in top of fuel tank. See FUEL SYSTEMS in REMOVAL, OVERHAUL & INSTALLATION article. If fuel pressure is as specified, go to step  6 .
    3. WARNING: High fuel pressure may be present in fuel lines. Open fuel system with caution. See FUEL SYSTEM PRESSURE RELEASE  .
  3. Turn ignition off. Release fuel pressure. Raise and support vehicle. Disconnect fuel pressure line from fuel pump module. Connect 5/16" Fuel Line Pressure Test Adapter Hose (6539) between disconnected fuel line and fuel pump module. Connect fuel pressure gauge to fuel line adapter hose. Turn ignition on. Using scan tool, actuate ASD fuel system test and note fuel pressure. Fuel pressure should be 44.2-54.2 psi (3.23-3.96 kg/cm2 ). If fuel pressure is within specification, stop ASD fuel system activation, release fuel pressure and replace fuel supply line. If fuel pressure is not within specification, stop ASD fuel system activation, and go to next step.
  4. Release fuel pressure. Remove fuel pump module and inspect fuel inlet strainer. If fuel inlet strainer is plugged, replace fuel pump inlet strainer. If fuel inlet strainer in not plugged, go to next step.
  5. At this time, fuel pump module is assumed to be defective. Replace fuel pump module.
  6. Turn ignition on. Using scan tool, read 1/1 HO2S voltage. If voltage is 0.4-0.6 volt, go to next step. If voltage is not 0.4-0.6 volt, go to step  13 .
  7. Turn ignition off. Wait at least 10 minutes to allow HO2S to cool. Turn ignition on. Using scan tool in SENSOR mode, monitor 1/1 HO2S voltage until voltage stabilizes at 0.4-0.6 volt. Using scan tool, actuate HO2S HEATER TEST and monitor 1/1 HO2S voltage for 3 minutes. If voltage does not stay at 0.4-0.6 volt, go to next step. If voltage stays at 0.4-0.6 volt, replace 1/1 HO2S.
  8. Ensure ignition is on. Ensure throttle plate is fully closed against stop. Using scan tool, read TP sensor voltage. If voltage is less than 0.92 volt, go to next step. If voltage is 0.92 volt or more, check throttle plate and linkage for binding. Repair throttle plate and linkage as necessary. If throttle plate and linkage are okay, replace TP sensor.
    9. NOTE: Throttle linkage must be moved very slowly while looking for a smooth change in voltage in this step.
  9. Ensure ignition is on. Using scan tool, read TP sensor voltage. Slowly open and close throttle plate while monitoring TP sensor voltage. If voltage change is smooth, go to next step. If voltage change is erratic, replace TP sensor.
  10. Turn ignition off. Connect a vacuum gauge to manifold vacuum source. Start engine and allow it to idle. If engine will not idle, maintain a constant RPM above idle. Using scan tool in SENSOR mode, monitor Manifold Absolute Pressure (MAP) vacuum value. Compare scan tool reading with vacuum gauge reading. If scan tool reading is within one in. Hg of vacuum gauge, turn ignition off and go to next step. If scan tool reading is not within one in. Hg of vacuum gauge reading, replace MAP sensor.
  11. Turn ignition on. Using scan tool, read ECT sensor value. If engine coolant temperature is more than 180°F (82°C), allow engine coolant temperature to cool down to 150°F (66°C) before proceeding. If engine coolant temperature is 150°F (66°C) or less, start engine and monitor scan tool until engine coolant temperature reaches 180°F (82°C). If ECT sensor value increases smoothly and reaches at least 180°F (82°C), go to next step. If ECT sensor value increase is erratic or does not reach at least 180°F (82°C), check for cooling system problems. Repair cooling system as necessary. If cooling system is okay, replace ECT sensor.
  12. Check the following additional items as possible mechanical problems:
    • Engine vacuum must be at least 13 in. Hg with shift selector in Neutral.
    • Engine valve timing must be within specification.
    • Engine compression must be within specification.
    • Engine exhaust system must be free of restrictions.
    • Engine Closed Crankcase Ventilation (CCV) system must flow freely.
    • Torque converter stall speed must be within specification.
    • Power brake booster must have no internal leaks.
    • Fuel must be free of contamination.
    • Fuel injectors must be free of restrictions.
    • Fuel injector control wires must be installed on correct injector.

    Repair any mechanical problems as necessary. If no mechanical problems exist, test is complete.

  13. Turn ignition on. Disconnect 1/1 HO2S connector. Using scan tool, read 1/1 HO2S voltage. If voltage is not 0.4-0.6 volt, go to next step. If voltage is 0.4-0.6 volt, replace 1/1 HO2S.
  14. Turn ignition off. Disconnect PCM connectors. PCM is located in engine compartment on left front fender, near radiator support. Measure resistance between ground and Black/Dark Green wire at 1/1 HO2S harness connector. If resistance is 5 ohms or more, go to next step. If resistance is less than 5 ohms, repair short to ground in Black/Dark Green wire between PCM and 1/1 HO2S.
  15. At this time, PCM is assumed to be defective. Replace PCM.
  16. At this time, conditions for DTC to set do not exist or fault is an intermittent problem. Start engine and allow it to idle. Ensure engine is at normal operating temperature. Using scan tool, monitor parameters related to DTC while wiggling connectors and wiring harness. If value changes while wiggling connectors and wiring harness, repair connectors and wiring harness where wiggling caused value to change. If value does not change, review FREEZE FRAME data using scan tool. Attempt to duplicate condition that caused DTC to set. Check for any related technical service bulletins that may apply. Visually inspect related connectors and wiring harness. Repair connectors and wiring harness as necessary. See WIRING DIAGRAMS article. If no problems are indicated, test is complete.
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