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DTC P0171: Fuel System Lean Or DTC P0172: Fuel System Rich

NOTE: If any DTCs besides DTC P0171 and P0172 are set, diagnose those DTCs first.
  1. Connect scan tool to Data Link Connector (DLC). Start engine and allow it to reach normal operating temperature. Increase engine speed to 2000 RPM and release to idle. Repeat 5-6 times continuously while monitoring HO2S No. 1 parameter on scan tool. If display reading varies from less than 0.3 volt to greater than 0.6 volt, go to next step. If display reading does not vary from less than 0.3 volt to greater than 0.6 volt, see DTC P0134  : HEATED OXYGEN SENSOR (HO2S) NO. 1 CIRCUIT/NO ACTIVITY DETECTED.
  2. Check fuel pressure. See FUEL SYSTEM in BASIC TESTING article. Fuel pressure should be 38-44 psi (2.7-3.1 kg/cm2 ) with key on and engine off and 28-34 psi (2.0-2.4 kg/cm2 ) with engine running at idle. If fuel pressure is as specified, go to next step. If fuel pressure is not as specified, see CIRCUIT TEST B-3: FUEL PRESSURE INSPECTION under CIRCUIT TESTS in BASIC TESTING article.
  3. Inspect fuel injectors and fuel system circuit. See CIRCUIT TEST B-1: FUEL INJECTOR CIRCUIT INSPECTION under CIRCUIT TESTS in BASIC TESTING article. Repair as necessary. Retest system. If fuel system is okay, go to next step.
  4. Ensure engine is cool. Disconnect purge hose from EVAP canister. Start and idle engine. Place a finger over end of disconnected hose. If no vacuum is present, go to next step. If vacuum is present, check EVAP control system. See EMISSION SYSTEMS & SUB-SYSTEMS in SYSTEM/COMPONENT TESTS article. Repair as necessary.
  5. Check MAP sensor performance. See DTC P0106  : MANIFOLD ABSOLUTE PRESSURE (MAP) SENSOR PERFORMANCE. Repair as necessary. If MAP sensor performance is okay, go to next step.
  6. Check ECT sensor performance. See ENGINE SENSORS & SWITCHES in SYSTEM/COMPONENT TESTS article. Repair as necessary. If ECT sensor performance is okay, go to next step.
  7. Check IAT sensor performance. See ENGINE SENSORS & SWITCHES in SYSTEM/COMPONENT TESTS article. Repair as necessary. If IAT sensor performance is okay, go to next step.
  8. Check TP sensor performance. Follow procedures in step 3) of DTC P0121  : THROTTLE POSITION (TP) CIRCUIT/RANGE PERFORMANCE PROBLEM. Repair as necessary. Retest system. If TP sensor performance is okay, go to next step.
  9. Disconnect PCV valve from valve cover. Start engine and let idle. Check for vacuum at PCV valve. If vacuum is not present, disconnect PCV valve from hose. If engine runs rough or stalls, replace PCV valve. If engine does not run rough or stall, check for restriction in PCV hose. Repair as necessary. If PCV system is okay, replace PCM.
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