LEMON Manuals: Even more car manuals for everyone: 1960-2025
Home >> Chevrolet >> 2002 >> Impala LS >> Repair and Diagnosis >> Engine Performance >> System >> Engine Control System Self-Diagnostics - 3.8L >> Diagnostic Tests >> DTC P0171: Fuel Trim System Lean >> Diagnostic Procedures
April 5, 2026: LEMON Manuals is launched! Read the announcement.

Diagnostic Procedures

NOTE: If any DTCs are set, other than P0171, diagnose those DTCs before continuing. See DIAGNOSTIC TROUBLE CODE DEFINITIONS .
  1. Perform Diagnostic System Check - Engine Controls. See DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS  under SELF-DIAGNOSTIC SYSTEM. After performing Diagnostic System Check - Engine Controls, go to next step.
  2. Install scan tool. Start and idle engine at normal operating temperature in closed loop. Record LTFT data. Turn engine off. Turn ignition on, engine off. Review FREEZE FRAME/FAILURE RECORDS, and record displayed data for this DTC. Does scan tool indicate LTFT is more than 23 percent? If yes, go to next step. If no, see DIAGNOSTIC AIDS .
  3. Operate engine at idle. Using scan tool, observe HO2S parameters. Does scan tool indicate HO2S is fluctuating to more than 800 millivolts and to less than 200 millivolts? If yes, go to next step. If no, go to step  5.
  4. Turn engine off. Visually and physically inspect vacuum hoses for splits, kinks, and proper connections. Ensure vehicle has sufficient fuel in tank. If fuel pressure is too low this DTC may set. Check for fuel contamination. Did you find and correct the condition? If yes, go to step  7. If no, go to step  6.
  5. Turn engine off. Check HO2S for proper installation. Check to ensure electrical connectors and wires are secured and not contacting exhaust system. Check for short between Purple wire and Tan wire between HO2S and PCM. Did you find and correct the condition? If yes, go to step  7. If no, see FUEL SYSTEMS in appropriate SYSTEM & COMPONENT TESTING article.
  6. Operate engine at idle. Check for missing, loose, or leaking exhaust components, forward of HO2S. Check for vacuum leaks at intake manifold, throttle body and injector "O" rings. Check air induction system and air intake ducts for leaks. Check AIR system for leaks, improper air delivery, and shutoff valve not closing. Check crankcase ventilation system for leaks. Did you find and correct the condition? If yes, go to next step. If no, diagnose engine mechanical problem.
    7. NOTE: After repairs, use scan tool FUEL TRIM RESET function to reset LTFT.
  7. Using scan tool, clear DTCs. Turn ignition off for 30 seconds. Start engine. Operate vehicle within Conditions for Running DTC. Does DTC run and pass? If yes, go to next step. If no, go to step  2.
  8. Using scan tool, observe stored information. Does scan tool displays any DTCs that have not been diagnosed? If yes, see DIAGNOSTIC TROUBLE CODE DEFINITIONS . If no, system is okay.
scientia non olet · About LEMON Manuals