P0401

Circuit Description
The powertrain control module (PCM) tests the exhaust gas recirculation (EGR) system during deceleration by momentarily commanding the EGR valve open while monitoring the manifold absolute pressure (MAP) sensor signal. When the EGR valve is opened, the PCM should detect a proportional increase in the MAP. If the expected increase in MAP is not detected, the PCM measures the amount of error that was detected and adjusts an internal fail counter towards a tail threshold level. When the fail counter exceeds the fail threshold level, the PCM sets DTC P0401. The number of test samples required to accomplish this may vary according to the amount of detected flow error.
Normally, the PCM only allows one EGR flow test sample to be taken during an ignition cycle. To aid in verifying a repair, the PCM allows 12 test samples during the first ignition cycle following a scan tool Clear Info function or a battery disconnect. Between 9 and 12 samples should be sufficient for the PCM to determine adequate EGR flow and pass the EGR test if the system is operating correctly.

Conditions for Running the DTC
1. None of the following DTCs are set:
^ Throttle position (TP) sensor DTCs
^ Vehicle speed sensor (VSS) DTCs
^ Intake air temperature (IAT) sensor DTCs
^ Manifold absolute pressure (MAP) sensor DTCs
^ EGR pintle position DTCs
^ Engine coolant temperature (ECT) sensor DTCs
^ Crankshaft position (CKP) sensor DTCs
^ Mass air flow (MAF) sensor DTCs
^ Idle speed DTCs
^ Misfire DTCs
^ Fuel injector fault DTCs
2. The BARO is greater than 70 kPa.
3. The ECT is greater than 75°C (167°F).
4. The IAT is greater than 80°C (176°F).
5. The engine speed is between 900 RPM and 1400 RPM.
6. The idle air control (IAC) position is steady.
7. The vehicle speed is more than 25 MPH.
8. The Throttle is closed.
9. The commanded state of the A/C relay is steady.

Conditions for Setting the DTC
The MAP changes monitored during the EGR flow test indicate insufficient EGR flow.

Action Taken When the DTC Sets
^ The PCM illuminates the malfunction indicator lamp (MIL) during the first trip in which the diagnostic test has run and failed.
^ The PCM stores the conditions which were present when the DTC set as Freeze Frame and Fail Records data.

Conditions for Clearing the MIL/DTC
^ The PCM turns the MIL off during the third consecutive trip in which the diagnostic has run and passed.
^ The history DTC clears after 40 consecutive warm-up cycles have occurred without a malfunction.
^ The DTC may be cleared by using the scan tool Clear Info function or by disconnecting the PCM battery feed.

Diagnostic Aids

Important: If the EGR valve shows signs of excessive heat, inspect the exhaust system for blockage. Refer to Restricted Exhaust System Check. Component Tests and General Diagnostics

To verify a repair, clear info with the scan tool and run the EGR flow test keeping the following items in mind:
^ The PCM only runs the EGR flow test during a gradual deceleration.
^ The PCM only runs the EGR test during a closed Throttle condition.
^ The PCM only runs the EGR test at vehicle speeds above 25 MPH.
^ Several deceleration cycles are necessary to run a sufficient number of EGR flow test samples (9-12).
^ The EGR Test Counter displayed on the scan tool may be useful in determining that the EGR flow test is running and to keep track of the number of test samples taken. The counter increments each time a test sample is taken.
Inspect for the following conditions:
^ Any vacuum restriction to the MAP sensor. A restriction to the MAP sensor may cause the EGR flow test to fail due to insufficient MAP changes being monitored during the test. Inspect for objects blocking the vacuum to the MAP sensor. Inspect for kinked or pinched MAP sensor vacuum hoses (if equipped.)
^ A faulty connection at the PCM or the EGR valve. Inspect the harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and faulty terminal to wire connections.
^ A damaged harness. Inspect the wiring harness for damage. If the harness appears to be OK observe the Actual EGR Position display on the scan tool while moving the connectors and wiring harnesses related to the EGR valve. A change in the display indicates the location of the malfunction.
Reviewing the Fail Records vehicle mileage since the diagnostic test last failed may help determine how often the condition that caused the DTC to be set occurs. This may assist in diagnosing the condition.