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DTC P0171 - Bank 1 System Too Lean

  1. Perform On-Board Diagnostic (OBD) System Check. See BASIC TESTING article. Read and record FREEZE FRAME and/or FAIL RECORDS data for each DTC set. Go to next step.
  2. If any DTCs other than P0171 and P0174 are set, diagnose them first before proceeding. If no other DTCs are set, go to next step.
  3. Operate vehicle in closed loop mode while using scan tool to read LT and ST FUEL TRIM values. If LT FUEL TRIM value is about 16 percent and ST FUEL TRIM value is about 20 percent, go to step 5). If values are not as specified, go to next step.
  4. Using scan tool, read and record FAIL RECORDS data. Clear DTCs and operate vehicle within FAILURE RECORDS conditions. If DTC P0171 is set, go to next step. If DTC P0171 is not set, check for an intermittent problem. See TESTS W/O CODES article.
  5. If DTC P0174 is also set, go to next step. If DTC P0174 is not set, go to step 17).
  6. Check vacuum hoses for splits, kinks and proper routing. If a problem is found, repair as necessary. Retest system. If no problem is found, go to next step.
  7. Check PCV valve for proper installation. If a problem is found, repair as necessary. Retest system. If no problem is found, go to next step.
  8. Check throttle body inlet screen for blockage or foreign objects which may partially block airflow sample through MAF sensor. If a problem is found, repair as necessary. Retest system. If no problem is found, go to next step.
  9. Start engine. If a high or unsteady idle condition exists, go to next step. If Idle is okay, go to step 12).
  10. Using scan tool, read IDLE AIR CONTROL (IAC) value. If value is greater than 5 counts, go to step 12). If value is 5 counts or less, go to next step.
  11. Check throttle body, intake manifold, and EGR valve for vacuum leaks. If a problem is found, repair as necessary. Retest system. If no problem is found, go to next step.
  12. Check IAC valve for proper operation. If a problem is found, repair as necessary. Retest system. If no problem is found, go to next step.
  13. Check for fuel contamination. If fuel is contaminated, repair as necessary. Retest system. If fuel is okay, go to next step.
  14. Check PCM injector, power and sensor ground circuits for clean and tight connections. If a problem is found, repair as necessary. Retest system. If no problem is found, go to next step.
  15. Disconnect MAF sensor harness connector. Operate vehicle in closed loop mode while monitoring scan tool ST FUEL TRIM. If ST FUEL TRIM decreases to about zero percent, go to step 22). If ST FUEL TRIM does not decrease to about zero percent, go to next step.
  16. Perform FUEL SYSTEM PRESSURE TEST under BASIC FUEL SYSTEM CHECKS in BASIC TESTING article. If a problem is found, repair as necessary. Retest system. If no problem is found, go to next step.
  17. Perform EVAP SYSTEM CHECK. See EMISSION SYSTEMS & SUB-SYSTEMS in SYSTEM/COMPONENT TESTS article. If a problem is found, repair as necessary. Retest system. If no problem is found, go to next step.
  18. Check intake manifold, injector "O" rings, EGR adapter, EGR valve and EGR feed pipes for vacuum leaks. If a problem is found, repair as necessary. Retest system. If no problem is found, go to next step.
  19. Check exhaust system for corrosion, loose or missing hardware. If a problem is found, repair as necessary. Retest system. If no problem is found, go to next step.
  20. Perform INJECTOR BALANCE TEST. See FUEL SYSTEM in SYSTEM/COMPONENT TESTS article. If a problem is found, repair as necessary. Retest system. If no problem is found, go to next step.
  21. Ensure HO2S is installed securely, and connector and harness are not contacting exhaust manifold or ignition wires. If a problem is found, repair as necessary. Retest system. If no problem is found, check for an intermittent problem. See TESTS W/O CODES article.
  22. Replace MAF sensor. Retest system.
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