GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Buick 🢒 2000 🢒 GL7 (Export) 🢒 DTC 🢒 DTC P0401 Exhaust Gas Recirculation (EGR) Flow Insufficient

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 sense a proportional increase in the MAP. If the expected increase in the MAP is not detected, the PCM notes the amount of error and adjusts an internal fail counter towards a fail 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 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 9 and 12 samples should be sufficient for the PCM to determine an adequate EGR flow and pass the EGR test.

Conditions for Running the DTC
    •  No TP, MAP, MAF, ECT, IAT, CKP, VSS, injector circuit, misfire, idle speed, or EGR pintle position DTCs are set.
    • The A/C status does not change.
    • The transaxle range does not change.
    • The TP angle is less than 1.5 percent (deceleration).
    • The ECT is more than 75°C (167°F).
    •  The engine speed is between 950 RPM and 1300 RPM.
    •  The IAC position is steady.
    • The MAP is steady between 15 and 70 kPa.
    •  The vehicle speed is more than 56 km/h (35 mph) during deceleration.
    • The BARO is more than 70 kPa
    • The vehicle will need to be driven at more than 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 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

The MAP changes monitored during the EGR flow test indicate an insufficient EGR flow.

Action Taken When the DTC Sets
    • The PCM will illuminate the malfunction indicator lamp (MIL) during the first trip in which the diagnostic test has been run and failed.
    • The PCM will store conditions which were present when the DTC set as Freeze Frame and Failure Records data.

Conditions for Clearing the MIL/DTC
    • The PCM will turn OFF the malfunction indicator lamp (MIL) during the third consecutive trip in which the diagnostic has 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

Notice: In order to prevent further damage if the EGR valve shows signs of excessive heat, check the exhaust system for blockage (possibly a plugged converter) using the procedure found on the restricted exhaust system check. If the exhaust system is restricted, repair the cause; one of which might be an injector which is open due to one of the following reasons:

   • Stuck
   • Grounded driver circuit
Check the oil for possible fuel contamination if a stuck open fuel injector is found.

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 of more than 48 km/h (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.

Inspect for the following conditions:

    •  A poor connection or 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 the wiring harnesses related to the EGR valve. A change in the display will indicate the location of the fault. Refer to Testing for Intermittent Conditions and Poor Connections in Wiring Systems.
    •  Reviewing the Failure Records vehicle mileage since the diagnostic test last failed may help determine how often the condition that caused the DTC to set occurs. This may assist in diagnosing the condition.

Step

Action

Values

Yes

No

1

Did you perform the Powertrain On-Board Diagnostic (OBD) System Check?

--

Go to Step 2

Go to Powertrain On Board Diagnostic (OBD) System Check

2
  1. Inspect the exhaust system for modification of the originally installed parts or any leaks.
  2. If a problem was found, repair the exhaust system as necessary.

Was a condition present that required repair?

--

Go to Step 5

Go to Step 3

3
  1. Remove the EGR valve.
  2. Visually and physically inspect the following items:
    3. •  The pintle, the valve passages and the adapter for excessive deposits or any kind of a restriction
    • The EGR valve gasket and the pipes for leaks
  3. If a problem is found, clean or replace any EGR system components as necessary.

Was a condition present that required repair?

--

Go to Step 5

Go to Step 4

4
  1. Remove the EGR inlet and outlet pipes from the exhaust manifold and the intake manifold.
  2. Inspect the manifold EGR ports and the EGR inlet and outlet pipes for a blockage caused by excessive deposits, casting flashing, or other damage.
  3. If a problem is found, correct the condition as necessary.

Was a condition present that required repair?

--

Go to Step 5

Go to Diagnostic Aids

5
  1. Review and record the scan tool Failure Records data.
  2. Clear the DTC and monitor the EGR Test Count display on the scan tool while operating the vehicle as specified in the Diagnostic Aids.
  3. Continue operating the vehicle EGR Test Count until 9-12 test samples have been taken.
  4. Select the scan tool Specific DTC information for DTC P0401 and note the test result.

Does the scan tool indicate DTC P0401 test Ran and Passed?

--

System OK

Go to Step 2