P0401

Circuit Description

The PCM tests the EGR system during deceleration by momentarily commanding the EGR valve to open while monitoring the Manifold Absolute Pressure (MAP) sensor signal. When the EGR valve is opened, the PCM should see a proportional increase in MAP. If the expected increase in MAP is not seen, the PCM notes the amount of error that was detected and adjusts an internal fail counter towards a fail threshold level. When the fail counter exceeds the fail threshold level, the PCM will set DTC P0401. The number of test samples required to accomplish this may vary according to the amount of detected flow error.

Normally, the PCM will only allow one EGR flow test sample to be taken during an ignition cycle. To aid in verifying a repair, the PCM allows twelve test samples during the first ignition cycle following a scan tool Clear Info or a battery disconnect. Between nine and twelve samples should be sufficient for the PCM to determine adequate EGR flow and pass the EGR test.

Conditions for Running the DTC

^ No TP, MAP, MAF, ECT, IAT, CKP sensor, VSS, injector circuit, Misfire, Idle speed, EGR Pintle Position, DTCs set.
^ The AC status does not change.
^ The transaxle range does not change.
^ TP angle is less than 1% (deceleration).
^ Engine coolant temperature is greater than 75°C (167°F).
^ Engine speed is between 1000 RPM and 1250 RPM.
^ IAC position is steady.
^ MAP steady between 15 and 70 kPa.
^ Vehicle speed is greater than 25 mph during deceleration.
^ Barometric Pressure (BARO) sensor greater than 70 kPa.
^ The vehicle will need to be driven above 50 mph, and then allowed to decelerate. When the vehicle is decelerating while meeting all of the criteria listed above, the PCM will enable the test to run. When the test runs, you will see the Desired EGR parameter and the Actual EGR position on the scan tool change from 0 to a calibrated value above 0.

Conditions for Setting the DTC

MAP changes monitored during the EGR flow test indicate insufficient EGR flow:

Action Taken When the DTC Sets

^ The PCM will illuminate the MIL during the first trip in which the diagnostic runs and fails.
^ If equipped with traction control, the PCM will command the Electronic Brake and Traction Control Module (EBTCM) via the serial data circuit to turn OFF traction control and illuminate the TRACTION OFF lamp.
^ The PCM will store conditions which were present when the DTC set as Freeze Frame and Fail Records data.

Conditions for Clearing the MIL/DTC

^ The PCM will turn the MIL OFF during the third consecutive trip in which the diagnostic has been run and passed.
^ The history DTC will clear after 40 consecutive warm-up cycles have occurred without a malfunction.
^ The DTC can be cleared by using a scan tool.

Diagnostic Aids

To verify a repair, clear info with the scan tool and run the EGR flow test keeping the following items in mind:

^ The PCM will only run the EGR flow test during a gradual deceleration.
^ The PCM will only run the EGR test during a closed throttle condition.
^ The PCM will only run the EGR test at vehicle speeds above 30 mph.
^ Several deceleration cycles will be necessary to run a sufficient number of EGR flow test samples (9-12).
^ The EGR Test Counter displayed on the scan tool can be useful in determining that the EGR flow test is running and to keep track of the number of test samples taken. The counter will increment each time a test sample is taken.

Check for the following conditions:

Important: Remove any debris from the connector surfaces before servicing a component. Inspect the connector gaskets when diagnosing or replacing a component. Ensure that the gaskets are installed correctly. The gaskets prevent contaminate intrusion.

^ Poor terminal connection.
Inspect the harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and faulty terminal to wire connection. Use a corresponding mating terminal to test for proper tension. Refer to Diagrams.
^ Damaged harness.
Inspect the wiring harness for damage. if the harness appears to be OK, observe the sensor display on the scan tool while moving connectors and wiring harnesses related to the sensor. A change in the sensor display may indicate the location of the fault.
^ Inspect the PCM and the engine grounds for clean and secure connections.

If the DTC is determined to be intermittent, reviewing the Fail Records can be useful in determining when the DTC was last set.