LEMON Manuals: Even more car manuals for everyone: 1960-2025
Home >> Toyota >> 1998 >> Avalon XLS >> Repair and Diagnosis >> Engine Performance >> System >> Engine Control System Tests W/Codes >> Self-Diagnostic Tests >> DTC P0401: Insufficient Exhaust Gas RECIRCULATION (EGR) Flow Detected >> Diagnosis & Repair (Using OBD-II Scan Tool)
April 5, 2026: LEMON Manuals is launched! Read the announcement.

Diagnosis & Repair (Using OBD-II Scan Tool)

  1. Disconnect EGR temperature sensor. Sensor is located on manifold between EGR valve and intake air chamber. Measure resistance between EGR temperature sensor terminals. If resistance is 2500-600,000 ohms, go to next step. If resistance is not 2500-600,000 ohms, replace EGR temperature sensor.
  2. Check for open or short circuit in wiring harness between EGR temperature sensor and ECM. See appropriate wiring diagram in WIRING DIAGRAMS article. Repair wiring as necessary. If wiring harness is okay, go to next step.
  3. Check all EGR system vacuum hoses. Replace hoses as necessary. If vacuum hoses are okay, access ECM behind glove box. Ensure ignition is off. On models with Calif. emissions and except Calif. emissions with engine immobilizer system and/or traction control, disconnect ECM E6 connector. See Figure. Connect a jumper wire between ground and terminal No. 18 (Black/White wire) at ECM E6 connector. Go to next step. On except Calif. emission models without engine immobilizer system and/or traction control, disconnect ECM E6 connector. See Figure. Connect a jumper wire between ground and terminal No. 12 (Black/White wire) at ECM E6 connector. Go to next step.
    NOTE: For information on engine immobilizer system, see COMPUTERIZED ENGINE CONTROLS in THEORY/OPERATION article.
  4. On all models, apply air pressure to EGR Vacuum Switching Valve (VSV) port "E". VSV is located under "V" bank cover on top of engine. With jumper wire connected, air should flow from port "F". See Figure. With jumper wire disconnected, air should flow from port "G". If VSV operates as specified, go to step 7). If VSV does not operate as specified, go to next step.
  5. Remove EGR VSV. Using ohmmeter, measure resistance between VSV terminals. Resistance should be 27-33 ohms at 68°F (20°C). Measure resistance between each terminal and body of VSV. Resistance should be infinite. If resistance is not as specified, replace VSV. If resistance is as specified, go to next step.
  6. Check VSV operation. Apply battery voltage to VSV terminals. Apply air to VSV port "E". Air should flow from port "F". Remove battery voltage from VSV terminals. Air should flow from port "G". Replace VSV as necessary. If VSV is okay, check for open in EGR VSV circuits between fuse block in left side of engine compartment and ECM. See appropriate wiring diagram in WIRING DIAGRAMS article. Repair as necessary.
  7. Perform EXHAUST GAS RECIRCULATION (EGR) SYSTEM TEST. See EMISSION SYSTEMS & SUB-SYSTEMS in appropriate SYSTEM/COMPONENT TESTS article. Replace components as necessary. If EGR system is functioning properly, check EGR Vacuum Control Valve (VCV). See appropriate SYSTEM/COMPONENT TESTS article. Replace as necessary. If VCV is okay, replace ECM.
scientia non olet · About LEMON Manuals