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DTC P0401: EGR System Failure

NOTE: For connector terminal ID, see CONNECTOR IDENTIFICATION  . For wiring diagram, see WIRING DIAGRAMS article.
NOTE: Monitored with engine temperature more than 180°F (82°C), ambient temperature more than 40°F (4°C), engine in closed loop, O2S operational, engine under a steady load with throttle open, and O2S controller at or near its limits. Set condition is when all conditions are met, EGR is turned off and on momentarily. O2S is monitored for changes in its output signal during this test. If no significant change or too much change is seen, a DTC is set. Possible causes are: exhaust leak, restricted intake manifold, EGR backpressure transducer defective, EGR valve defective, EGR valve diaphragm defective, EGR valve incorrectly mounted, EGR transducer line blocked or vacuum supply problem to EGR solenoid.
  1. Turn ignition on with engine off. Using scan tool, read DTCs. If GLOBAL GOOD TRIP displayed is zero, go to step  3 . If GLOBAL GOOD TRIP is not zero, go to next step.
  2. At this time the EGR system failure does not exist or is an intermittent problem. Using scan tool, read FREEZE FRAME data. Attempt to duplicate the condition that causes DTC to be set. While using FREEZE FRAME data, pay attention to the conditions such as vehicle speed, temperature, load and MAP sensor vacuum. If EGR system failure reoccurs, go to next step. If EGR system failure does not reoccur, misfire no longer is present. Perform TEST VER-5A: ROAD TEST FOR OBD II VERIFICATION  .
  3. Turn off all accessories. Start engine and allow to reach normal operating temperature. Disconnect EGR solenoid vacuum supply hose. Connect a vacuum gauge to disconnected hose. If vacuum is more than 10 in. Hg, go to next step. If vacuum is 10 in. Hg or less, repair vacuum supply to EGR solenoid. Perform TEST VER-5A: ROAD TEST FOR OBD II VERIFICATION  .
  4. Reconnect vacuum hose to EGR solenoid. Disconnect EGR valve vacuum hose. Connect a vacuum gauge to disconnected hose. If vacuum gauge pulsates, go to next step. If vacuum gauge does not pulsate, go to step  7 .
  5. While observing vacuum gauge, momentarily raise engine speed to more than 2000 RPM. If vacuum gauge stabilized, go to next step. If vacuum gauge did not stabilize, replace EGR valve assembly. Perform TEST VER-5A: ROAD TEST FOR OBD II VERIFICATION  .
  6. Remove EGR valve and inspect manifold and tube for restrictions. If any restrictions are present, repair or replace manifold and tube as necessary. Perform TEST VER-5A: ROAD TEST FOR OBD II VERIFICATION  . If restrictions are not present, replace EGR valve assembly. Perform TEST VER-5A: ROAD TEST FOR OBD II VERIFICATION  .
  7. Inspect exhaust system for a sizeable leak. If a leak was not found, go to next step. If a leak was found, repair as necessary. Perform TEST VER-5A: ROAD TEST FOR OBD II VERIFICATION  .
  8. Reconnect hose to EGR valve. Disconnect backpressure hose from transducer. Connect a pressure gauge to backpressure hose. If pressure gauge is not pulsating, go to next step. If pressure gauge is pulsating, replace EGR assembly. Perform TEST VER-5A: ROAD TEST FOR OBD II VERIFICATION  .
  9. Inspect EGR mounting. Check valve tightness and valve gasket. If any problems were identified, repair as necessary. Perform TEST VER-5A: ROAD TEST FOR OBD II VERIFICATION  . If any problems were not identified, replace EGR valve assembly. Perform TEST VER-5A: ROAD TEST FOR OBD II VERIFICATION  .
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