GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Chevrolet 🢒 2008 🢒 Vivant 🢒 HVAC 🢒 HVAC - Automatic 🢒 DTC P0400, P0401, P0402, or P1402

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 Descriptors

DTC P0401: Exhaust Gas Recirculation (EGR) Flow Insufficient

DTC P0402: Exhaust Gas Recirculation (EGR) Flow Excessive

Circuit Description

The engine control module (ECM) performs a insufficient flow test on the exhaust gas recirculation (EGR) system during deceleration and an excessive flow test during engine crank.

The insufficient flow test is when the ECM momentarily commanding the EGR valve to open while monitoring the signal of the manifold absolute pressure (MAP) sensor and the EGR position sensor. If the MAP signal is incorrect for the EGR pintle position, 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 tests required to exceed the fail threshold may vary according to the amount of detected EGR flow error. The ECM will only allow one EGR flow test during an ignition cycle. Following a code clear event, the ECM will perform multiple EGR flow tests.

The excessive flow test is when the ECM monitors the position of the EGR valve during engine crank. The EGR valve should not be open during engine crank.

Conditions for Running the DTC

P0401

    • DTCs P0106, P0107, P0108, P0112, P0113, P0117, P0118, P0122, P0123, P0201, P0202, P0203, P0204, P0351, P0352, P0402, P0404, P0405, P0406, P0502, P0506, P0507 and P1404 are not set.
    • The EGR flow test is ran in decel fuel cut-off (DFCO) mode with the following conditions present before deceleration occurs:
       - The A/C compressor clutch does not change state during deceleration.
       - The barometric pressure (BARO) is greater than 72 kPa.
       - The MAP is between 10-37 kPa.
       - The vehicle speed is greater than 18 km/h (29 mph) before deceleration.
    • The EGR flow test is ran when the following conditions are met during DFCO mode:
       - The EGR position is less than 1 percent.
       - The engine speed is between 1,400-3,000 RPM.
       - The MAP does not vary greater than 1 kPa.
       - The throttle position does not vary greater than 1 percent.
    • The EGR flow test is performed once per ignition cycle.
    • The DTC runs multiple times on the first ignition cycle after the DTC is cleared from the ECM memory.

P0402

    • The engine is cranking.
    • The ignition voltage is between 10-16 volts.
    • The DTC runs continuously when the above conditions are met .

Conditions for Setting the DTC

P0401

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

P0402

The ECM detects excessive EGR flow due to the EGR valve pintle being open greater than 55 percent during crank.

Action Taken When the DTC Sets
    • DTC P0401 is a Type C DTC.
    • DTC P0402 is a Type B DTC.

Conditions for Clearing the MIL/DTC
    • DTC P0401 is a Type C DTC.
    • DTC P0402 is a Type B DTC.

Reference Information

Schematic Reference

Engine Controls Schematics

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. Engine running, observe the DTC information with a scan tool. DTCs P0106, P0107, P0108, P0402, P0405, P0406, and P1404 should not set.
    2. ⇒If any DTCs are set, refer to Diagnostic Trouble Code (DTC) List - Vehicle .
  2. Ignition ON, observe the scan tool EGR Position Sensor parameter. The reading should be between 0-3 percent.
  3. Operate the vehicle within the Conditions for Running the DTC to verify the DTC does not reset. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records data.

Circuit/System Testing
  1. Verify none of the following conditions exist:
    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.
    • Leaks or restrictions at the MAP sensor.

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

    • Leaking, restricted, or modified exhaust system. Refer to Symptoms - Engine Exhaust .
    • Mechanical failure in the engine such as worn piston rings, camshaft or incorrect timing belt installation. Refer to Symptoms - Engine Mechanical .
  2. Ignition OFF, disconnect the harness connector at the EGR valve.
  3. Ignition OFF, connect a test lamp between the control circuit terminal A and 12 volts.
    4. ⇒If test lamp illuminates, test the control circuit for a short to ground. If the circuit tests normal, replace the ECM.
  4. If all circuits test normal, test or replace the EGR valve.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

    •  Exhaust Gas Recirculation Valve Replacement
    •  Control Module References for engine control module replacement, setup, and programming