GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2006 🢒 TopKick C-Series (Conventional) C6/C7/C8 🢒 Restraints 🢒 SIR 🢒 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/System Description

The engine control module (ECM) tests the exhaust gas recirculation (EGR) flow when the EGR is in operation. EGR desired flow is compared to EGR actual flow. If EGR flow is lower than expected DTC P0401 will set.

Conditions for Running the DTC
    • DTCs P0101, P0102, P0103, P0403, P0405, P0406 are not set.
    • The engine is running.
    • The EGR valve is actively being controlled.
    • DTC P0401 runs continuously when the above conditions are met.

Conditions for Setting the DTC

The air flow deviation monitored by the ECM during the EGR operation indicates insufficient EGR flow for 15 seconds.

Action Taken When the DTC Sets

    • The control module illuminates the malfunction indicator lamp (MIL) on the second consecutive ignition cycle that the diagnostic runs and fails.
    • The control module records the operating conditions at the time the diagnostic fails. The first time the diagnostic fails, the control module stores this information in the Failure Records. If the diagnostic reports a failure on the second consecutive ignition cycle, the control module records the operating conditions at the time of the failure. The control module writes the operating conditions to the Freeze Frame and updates the Failure Records.

Conditions for Clearing the DTC
    • The control module turns OFF the 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.

Reference Information

Schematic Reference

Engine Controls Schematics

Connector End View Reference

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

Scan Tool Reference

    •  Scan Tool Data List
    •  Scan Tool Data Definitions
    •  Scan Tool Output Controls

Circuit/System Verification

    Important: If you cannot duplicate the condition, 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.

  1. If DTCs P0101, P0403, P0405, P0406, P0698, or P0699 are set diagnose these DTCs.
  2. Using the output controls on the scan tool command the EGR solenoid to 10 percent.
    3. ⇒If the actual EGR position remains stationary, replace the EGR valve.
  3. Inspect the exhaust system for leaks, restrictions, or for modification of the original equipment manufacture (OEM) part.
  4. Visually inspect for leaks between the MAF sensor and the turbocharger.
  5. Visually inspect for a restriction in the air cleaner or the air intake ducts.
  6. Visually inspect for a restriction in the turbocharger charge air cooler.
  7. Visually inspect for a restriction in the intake manifold.
  8. Engine running, observe the DTC information with a scan tool. DTC P0401 should not set.
  9. 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

Inspect the charge air cooler for leaks. Refer to Charge Air Cooler Diagnosis .

⇒If no problem is found, perform the component testing.

Component Testing

Important: Perform the Circuit/System Verification before proceeding with the Component Testing.

  1. Using the output controls on the scan tool command the EGR solenoid to 10 percent.
    2. ⇒If the actual EGR position remains stationary, replace the EGR valve.
  2. Spray the EGR valve gaskets and EGR cooler gaskets with a soap solution. Start the engine. Command the turbocharger vane position control solenoid ON to 100 percent and inspect for leaks around the EGR valve gaskets.
    3. ⇒If a leak is found replace the EGR valve gasket.
  3. Remove the EGR valve. Refer to Exhaust Gas Recirculation Valve Replacement and inspect that the EGR is not stuck, that there are no leaks or restrictions in the EGR valve and EGR passages, and that there is no debris or damage that can cause the EGR valve to stick partially open or partially closed.
    4. ⇒If the EGR valve is sticking, or is damaged, replace the EGR valve.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

Exhaust Gas Recirculation Valve Replacement