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DTC P0171: 1/1 Fuel System Lean Or DTC P0174: 2/1 Fuel System Lean: Testing

  1. Turn ignition on. Using scan tool, read DTCs. If GLOBAL GOOD TRIP or FUEL SYSTEM LEAN GOOD TRIP counter is displayed and displayed count is "0", go to next step. If GLOBAL GOOD TRIP or FUEL SYSTEM LEAN GOOD TRIP counter is not displayed and/or displayed count is not "0", go to step  8.
    2. NOTE: Throttle linkage must be moved very slowly while looking for a smooth change in voltage in this step.
  2. Ensure ignition is on. Using scan tool, read TP sensor voltage while slowly opening and closing throttle plate. If voltage change is smooth, go to next step. If voltage change is erratic, replace TP sensor.
  3. Turn ignition on. Using scan tool, read TP sensor voltage. Ensure throttle is closed against throttle stop. If voltage is .92 volt or less, no problem is indicated at this time. Test is complete. If voltage is more than .92 volt, check for binding throttle. Repair or replace throttle body/linkage as necessary. If throttle is not binding, replace TP sensor.
    NOTE: To diagnose the following step, thermostat is assumed to be functioning properly. Manufacturer also recommends allowing vehicle to sit overnight in order to have a totally cold engine.
  4. Turn ignition on. Using scan tool, read ENG COOLANT TPM DEG value. If engine was allowed to sit overnight, scan tool temperature value should be near ambient air temperature. If engine coolant temperature is more than 180°F (82.2°C), allow engine coolant temperature to cool down to 150°F (65.5.°C) before proceeding. If engine coolant temperature is 150°F (65.5°C) or less, start engine. Using scan tool, monitor ENG COOLANT TPM DEG until engine coolant temperature reaches 180°F (82.2°C). Temperature value should increase smoothly from start-up to normal operating temperature and temperature should reach at least 180°F (82.2°C). If temperature value increased smoothly and temperature reached at least 180°F (82.2°C), go to next step. If temperature value did not increase smoothly and/or temperature did not reach at least 180°F (82.2°C), replace ECT sensor.
  5. Turn ignition off. Check fuel pump pressure and fuel pump volume. See TEST NTC-7: CHECKING FUEL DELIVERY . If fuel pump pressure and fuel pump volume are as specified, go to next step. If fuel pump pressure or fuel pump volume is not as specified, repair fuel system as necessary.
  6. 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 MAP vacuum value. Compare scan tool reading with vacuum gauge reading. If scan tool reading is within one in. Hg of vacuum gauge, go to next step. If scan tool reading is not within one in. Hg of vacuum gauge reading, replace MAP sensor.
  7. 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 Positive Crankcase Ventilation (PCV) 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.

  8. At this time, conditions for DTC to set do not exist or fault is an intermittent problem. Using scan tool, read and record FREEZE FRAME data and SIMILAR CONDITIONS WINDOW data. Road test vehicle and attempt to operate vehicle under conditions in FREEZE FRAME data and SIMILAR CONDITIONS WINDOW data, paying particular attention to fault setting conditions such as speed, temperature, load and MAP vacuum. Using scan tool, read DTCs. If DTC P0171 or P0174: FUEL SYSTEM LEAN is present, go to step 2. If DTC P0171 or P0174: FUEL SYSTEM LEAN is not present, no problem is indicated at this time. Check for any related technical service bulletins that may apply.
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