Diagnostic Aids

An intermittent may be caused by a poor connection, rubbed-through wire insulation or a broken wire inside insulation. Inspect harness connectors for backed-out terminals, improper mating, broken locks, improperly formed or damaged terminals, poor terminal-to-wire connection, and damaged harness. If DTC cannot be duplicated, check scan tool FAILURE RECORDS to determine mileage since last DTC was set.

1. Perform On-Board Diagnostic (OBD) System Check. See BASIC DIAGNOSTIC PROCEDURES 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 BANK 1 L.T. FUEL TRIM values. If L.T. FUEL TRIM value is more than 20 percent, go to step  5. If value is 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 P0174 is set, go to next step. If DTC P0174 is not set, check for an intermittent problem. See DIAGNOSTIC AIDS .
5. If DTC P0171 is also set, go to next step. If DTC P0171 is not set, go to step  15.
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 MAF 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  11.
10. 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.
11. Check for fuel contamination. If fuel is contaminated, repair as necessary. Retest system. If fuel is okay, go to next step.
12. 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.
13. Disconnect MAF sensor harness connector. Operate vehicle in closed loop mode while monitoring scan tool BANK 1 S.T. FUEL TRIM. If S.T. FUEL TRIM does not decrease to about zero percent, go to next step. If S.T. FUEL TRIM decreases to about zero percent, go to step  19.
14. Perform FUEL SYSTEM PRESSURE TEST under BASIC FUEL SYSTEM CHECKS in BASIC DIAGNOSTIC PROCEDURES article. If a problem is found, repair as necessary. Retest system. If no problem is found, go to next step.
15. 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.
16. 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.
17. Perform injector balance test. See FUEL SYSTEM in SYSTEM & COMPONENT TESTING article. If a problem is found, repair as necessary. Retest system. If no problem is found, go to next step.
18. 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 DIAGNOSTIC AIDS .
19. Replace MAF sensor. Retest system.