DTC P0401
Circuit Description
The powertrain control module (PCM) tests the exhaust gas recirculation (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 this diagnostic trouble code (DTC). 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 12 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
DTCs P0101, P0102, P0103, P0121, P0122, P0123, P0107, P0108, P0112, P0113, P0502, P0503, P0117, P0118, P0506, P0507, P1404. P0403, P0404, P0405, P0201-P0206, P0300, P0336, P1374, P1121, P1122, P1112, P1106, P1107, P1114, or P1115 are not set.
The system voltage is between 11-18 volts.
The AC status does not change.
The transaxle range does not change.
The throttle position (TP) angle is less than 1 percent, deceleration.
The intake air temperature is less than 80°C (176°F).
The engine coolant temperature (ECT) is greater than 75°C (167°F).
The engine speed is between 1,100-1,400 RPM.
The idle air control (IAC) position is steady.
The vehicle speed is greater than 45 km/h (28 mph) during deceleration.
The barometric pressure (BARO) sensor greater than 70 kPa (10.1 psi).
The vehicle will need to be driven above 80 km/h (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 is running, you will see the desired EGR parameter and the EGR position sensor on the scan tool change from 0 to a calibrated value above 0.

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

Action Taken When the DTC Sets
The control module illuminates the malfunction indicator lamp (MIL) when the diagnostic runs and fails.
The control module records the operating conditions at the time the diagnostic fails. The control module stores this information in the Freeze Frame/Failure Records.
The PCM disables the EGR valve for the ignition cycle.
Conditions for Clearing the MIL/DTC
The control module turns OFF the malfunction indicator lamp (MIL) after 3 consecutive ignition cycles that the diagnostic runs and does not fail.
A current DTC, Last Test Failed, clears when the diagnostic runs and passes.
A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic.
Clear the MIL and the DTC with a scan tool.

Diagnostic Aids

IMPORTANT
Inspect for the following conditions:

An EGR valve that shows signs of excessive heat--Ceck the exhaust system for blockage, or for a plugged catalytic converter. Refer to Restricted Exhaust in Engine Exhaust.
Vacuum restriction to the MAP sensor--A restriction to the MAP sensor can cause the EGR flow test to fail due to insufficient MAP changes being monitored during the test.
An engine that is running poorly may cause this DTC to set.
A poor connection or damaged harness--Inspect the wiring harness for damage.
If the harness appears to be OK, observe the EGR position sensor display on the scan tool while moving connectors and wiring harnesses related to the EGR valve. A change in the display indicates the location of the malfunction.

In order 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 40 km/h (28 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.

Test Description
The number below refers to the step number on the diagnostic table.

This DTC is set when the PCM does not detect a predetermined change in MAP when the EGR valve is commanded. This step checks for any MAP sensor DTCs. A malfunctioning MAP sensor may cause this DTC to set.