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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 2.5L and 4.0L Federal models, 1/1 HO2S is located in exhaust downpipe in front of catalytic converter. On 4.0L Calif. models, 1/1 HO2S is located in front exhaust manifold.
  1. Turn ignition on, with engine off. Using scan tool, read DTCs. If FUEL SYSTEM LEAN GOOD TRIP counter is displayed and displayed count is "0", go to step  3 . If FUEL SYSTEM LEAN GOOD TRIP counter is not displayed and/or displayed count is not "0", go to next step.
  2. 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. Road test vehicle under conditions in FREEZE FRAME data, paying particular attention to fault setting conditions such as speed, temperature, load and MAP vacuum. Using scan tool, read DTCs. If DTC P0171: FUEL SYSTEM LEAN is present, go to next step. If DTC P0171: FUEL SYSTEM LEAN is not present, no problem is indicated at this time. Test is complete.
  3. Ensure ignition is on, with engine off. Using scan tool, read ECT sensor value. If engine coolant temperature is more than 180°F (82.2°C), allow engine coolant temperature to cool down to 150°F (65.6°C) before proceeding. If engine coolant temperature is 150°F (65.6°C) or less, start engine and monitor scan tool until engine coolant temperature reaches 180°F (82.2°C). If ECT sensor value increases smoothly, go to next step. If ECT sensor value increase is erratic, check for cooling system problems and replace ECT sensor.
  4. Using scan tool, actuate HO2S HEATER TEST and monitor 1/1 HO2S voltage for 3 minutes. If voltage is more than .1 volt, replace 1/1 HO2S. If voltage is .1 volt or less, go to next step.
  5. Perform fuel pressure test. See FUEL PRESSURE TEST under FUEL SYSTEMS in BASIC DIAGNOSTIC PROCEDURES article. If fuel pressure is as specified, go to next step. If fuel pressure is not as specified, repair fuel system as necessary.
  6. 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.

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