GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Buick 🢒 2007 🢒 Lucerne 🢒 DTC 🢒 DTC P0401

Diagnostic Instructions
    • Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.
    • Review Strategy Based Diagnosis for an overview of the diagnostic approach.
    •  Diagnostic Procedure Instructions provides an overview of each diagnostic category.

DTC Descriptor

DTC P0401: Exhaust Gas Recirculation (EGR) Flow Insufficient

Circuit Description

The control module tests the exhaust gas recirculation (EGR) system during deceleration. The control module does this by momentarily commanding the EGR valve to open while monitoring the signal circuit of the manifold absolute pressure (MAP) sensor. When the EGR valve is opened, the control module will expect to see a predetermined increase in MAP. If the expected increase in MAP is not detected, the control module records the amount of MAP difference that was detected and adjusts a calibrated fail counter towards a calibrated fail threshold level. The number of EGR flow test counts required to exceed the fail threshold may vary according to the amount of detected EGR flow error.

The EGR valve position sensor is monitored by the control module. The 5-volt reference circuit, the low reference circuit, and the EGR valve position signal circuit are used by the control module to determine the EGR valve position. The control module compares the EGR Position Sensor parameter with the desired EGR Position parameter when the valve is commanded open or closed.

The control module controls the EGR valve with a solid state device called a driver. The driver supplies the EGR solenoid with 12 volts that is pulse width modulated (PWM) through the EGR solenoid high control circuit. A ground path is provided by the control module through the EGR solenoid low control circuit. The driver has the ability to detect an electrical malfunction on the EGR solenoid control circuits.

When the ignition switch is turned ON, the control module records the EGR Learned Minimum Position. The control module compares the EGR Learned Minimum Position parameter to the EGR Position Sensor parameter.

The control module will only allow one EGR flow test during an ignition cycle. To aid in verifying a repair, the control module will allow between 9 and 16 EGR flow test counts during the first ignition cycle following a code clear event.

Conditions for Running the DTC
    • DTCs P0068, P0101, P0102, P0103, P0106, P0107, P0108, P0112, P0113, P0117, P0118, P0120, P0201, P0202, P0203, P0204, P0205, P0206, P0220, P0300, P0335, P0336, P0403, P0404, P0405, P0406, P042E, P0502, P0503, P0506, P0507, P0604, P0606, P0641, P0651, P1106, P1107, P1125, P2101, P2108, P2135, U0107 are not set.
    • The engine run time may need to be more than 3 minutes.
    • The Ignition 1 Signal parameter is between 11-18 volts.
    • The AC Relay Command parameter does not change.
    • The EGR position is less than 1 percent.
    • The transmission is in third or fourth gear.
    • The Current Gear parameter does not change.
    • The decel fuel cut-off (DFCO) mode is not active.
    • The Intake Air Temperature (IAT) sensor parameter is between 7-100°C (41-212°F).
    • The Engine Coolant Temperature (ECT) sensor parameter is between 75-150°C (167-302°F).
    • The Engine Speed parameter is between 1,000-1,500 RPM.
    • The Manifold Absolute Pressure (MAP) sensor parameter is between 17-43 kPa.
    • The Barometric Pressure (BARO) parameter is more than 74 kPa.
    • The Throttle Position (TP) sensor parameter is less than 1 percent.
    •  The Vehicle Speed sensor parameter is between 45-113 km/h (28-70 mph) during deceleration.
    • The vehicle will need to be driven 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 EGR flow test to run. As the EGR flow test is running, you will see the desired EGR Position parameter and the EGR Position Sensor parameter on the scan tool momentarily change from 0 to a calibrated value above 0. Additionally, the EGR Flow Test Count parameter on the scan tool will increment when each EGR flow test is completed.

Conditions for Setting the DTC

The MAP changes monitored by the PCM during the EGR flow tests indicate an insufficient amount of EGR flow.

Action Taken When the DTC Sets

DTC P0401 is a Type A DTC.

Conditions for Clearing the MIL/DTC

DTC P0401 is a Type A DTC.

Reference Information

Schematic Reference

Engine Controls Schematics

Description and Operation

Exhaust Gas Recirculation (EGR) System Description

Connector End View Reference

    •  Powertrain Control Module Connector End Views
    •  Engine Controls Connector End Views

Electrical Information Reference

    •  Circuit Testing
    •  Connector Repairs
    •  Testing for Intermittent Conditions and Poor Connections
    •  Wiring Repairs

DTC Type Reference

Powertrain Diagnostic Trouble Code (DTC) Type Definitions

Scan Tool Reference

Scan Tool Data List

Circuit/System Verification

Important: If DTC P0641 or P0651 are also set, diagnose those DTCs first.

    • With the ignition ON and the engine OFF, command the EGR from 0-90 percent. The EGR Position Sensor parameter should remain within 3 percent of the Desired EGR Position parameter through the entire range.
    • Operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that are captured in the Freeze Frame/Failure Records List.

Circuit/System Testing

Important: You must complete the Circuit/System Verification before proceeding with Circuit/System Testing.

  1. Ignition OFF, disconnect the EGR valve harness connector.
  2. Ignition ON, connect a test lamp between B+ and the EGR low control circuit. The test lamp should illuminate.
    3. ⇒If the test lamp does not illuminate, test the EGR low control circuit for an open/high resistance. If the circuit tests normal, replace the ECM.
  3. Connect a test lamp between the EGR high control circuit and ground. Command the EGR valve from 0-90 percent and exit the EGR Solenoid Output Control. The test lamp should turn ON when commanded between 10-90 percent and turn OFF when commanded to 0 percent.
    4. ⇒If the test lamp is ON when commanded to 0 percent, test the EGR high control circuit for a short to voltage. If the circuit tests normal, replace the ECM.
    ⇒If the test lamp is always OFF while commanding the EGR valve from 0-90 percent, test the EGR high control circuit for an open, high resistance, or a short to ground. If the circuit tests normal, replace the ECM.
  4. If all circuits test normal, replace the EGR valve.

Repair Instructions
    •  Control Module References for control module replacement and programming
    •  Exhaust Gas Recirculation Valve Replacement

Repair Verification
  1. Clear the DTCs with a scan tool.
  2. Turn OFF the ignition for 30 seconds.
  3. Start the engine.
  4. Operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records.
  5. Observe the EGR Flow Test Count parameter on the scan tool. EGR Flow Test counts may need to be as many as 16 to pass this DTC after the DTCs have been cleared.