GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Buick 🢒 2008 🢒 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 provide an overview of each diagnostic category.

DTC Descriptor

DTC P0401: Exhaust Gas Recirculation (EGR) Flow Insufficient

Circuit/System Description

The engine control module (ECM) tests the exhaust gas recirculation (EGR) system during deceleration. The ECM 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 ECM will expect to see a predetermined increase in MAP. If the expected increase in MAP is not detected, the ECM 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.

Normally, the ECM will only allow one EGR flow test during an ignition cycle. To aid in verifying a repair, the ECM will allow up to 27 EGR flow test counts during the first ignition cycle following a code clear event. Between 8-14 EGR flow test counts should be sufficient for the ECM to determine adequate EGR flow and pass the EGR flow test. If the ECM detects an EGR flow error, DTC P0401 sets.

Conditions for Running the DTC
    • DTCs P0016, P0101, P0102, P0103, P0106, P0107, P0108, P0112, P0113, P0116, P0117, P0118, P0120, P0122, P0123, P0128, P0201, P0202, P0203, P0204, P0205, P0206, P0207, P0208, P0220, P0222, P0223, P0300, P0301-P0308, P0315, P0335, P0336, P0340, P0341, P0403, P0404, P0502, P0503, P0601, P0602, P0603, P0604, P0606, P060D, P062F, P0641, P0651, P1404, P2135 are not set.
    • The Ignition 1 signal parameter is between 11-18 volts.
    • The Current Gear parameter does not change.
    • The Intake Air Temperature (IAT) sensor parameter is less than 100°C (212°F).
    • The Engine Coolant Temperature (ECT) sensor parameter is between 5-150°C (41-230°F).
    • The Engine Speed parameter is between 1,000-1,800 RPM.
    • The Manifold Absolute Pressure (MAP) sensor parameter is between 11-50 kPa.
    • The Mass Air Flow (MAF) sensor parameter does not change more than 2 grams per second.
    • The Barometric Pressure (BARO) parameter is more than 74 kPa.
    • The Throttle Position (TP) sensor parameter is less than 1.19 percent.
    •  The Vehicle Speed sensor parameter is more than 37 km/h (23 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 ECM 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.
    • DTC P0401 runs once per trip typically, or 7 tests will be run per trip until 42 tests have been completed.

Conditions for Setting the DTC

The MAP changes monitored by the ECM during the EGR flow tests indicate an insufficient amount of EGR flow for more than 2.8 seconds.

Action Taken When the DTC Sets

DTC P0401 is a Type A DTC.

Conditions for Clearing the DTC

DTC P0401 is a Type A DTC.

Reference Information

Schematic Reference

Engine Controls Schematics

Connector End View Reference

Component 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

Control Module References for scan tool information

Circuit/System Verification
  1. If DTCs P0107, P0108, P0403, P0404, P0405, or P0406 are also set, refer to Diagnostic Trouble Code (DTC) List - Vehicle for further diagnosis.
  2. 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 data.

Circuit/System Testing
  1. Verify the following conditions are not present:
    2. • Incorrect EGR valve for engine application
    • A vacuum leak between the EGR valve and the intake manifold
    • Signs of external leakage usually indicated by carbon buildup around the mating surfaces of the components or an audible exhaust noise
    • Restrictions in the passages or EGR valve caused by carbon deposits or casting flash
    • MAP sensor for being shifted, stuck, skewed, and for correct vacuum supply

       Important: Restrictions can cause excessive back pressure and low engine vacuum.

    • Leaking, restricted, or modified exhaust system
    • Mechanical failure in the engine such as worn piston rings, camshaft, or incorrect timing components installation--Refer to Symptoms - Engine Mechanical .
  2. If all conditions test normal, replace the EGR valve. Refer to Exhaust Gas Recirculation Valve Replacement .

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.