GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2002 🢒 GMC C Sierra - 2WD 🢒 DTC 🢒 DTC P0401

Circuit Description

The powertrain control module (PCM) tests the exhaust gas recirculation (EGR) system during deceleration by momentarily commanding the EGR valve open. The pressure in the intake manifold increases when the EGR valve is open. The PCM monitors the manifold absolute pressure (MAP) sensor signal during the EGR system diagnostic in order to determine the amount of EGR flow. The PCM calculates an error based on the difference between the actual MAP increase and a predetermined value. The PCM records the error and adjusts an internal fail counter towards the fail threshold if the error is too great. The PCM sets this diagnostic trouble code (DTC) when the fail counter exceeds the fail threshold. The number of test samples required to exceed the fail threshold varies according to the magnitude of the detected flow errors.

The PCM allows only one EGR flow test sample to be taken during an ignition cycle. Following a scan tool Clear Info or a battery disconnect, the PCM allows more than one sample per ignition cycle until 13 samples have been obtained in order to aid in verifying a repair.

Conditions for Running the DTC
    • DTCs P0106, P0107, P0108, P0122, P0113, P0116, P0117, P0118, P0121, P0122, P0123, P0300, P0404, P0405, P0502, P0503, P0506, P0507, P1111, P1112, P1114, P1115, P1121, P1122, P1404, P1635, or P1639 are not set.
    • The ignition voltage is between 11.7-18 volts.
    • The barometric pressure (BARO) is more than 70 kPa.
    • The engine coolant temperature (ECT) is between 60-117°C (140-243°F).
    • The intake air temperature (IAT) is less than 65°C (140°F).
    • The engine speed is between 725-2,000 RPM.
    • The throttle position (TP) sensor angle is less than 1 percent.
    • The altitude compensated MAP is between 10-60 kPa (3-6 psi)
    • The engine vacuum is between 60-83 kPa (9-12 psi).
    • The idle air control (IAC) position is steady.
    • The vehicle speed is between 42-113 km/h (26-70 mph).
    • The change in vehicle speed sensor (VSS) is less 6 km/h (4 mph).
    • The change in MAP is less than 0.8 kPa.
    • The A/C clutch status is steady.
    • The fuel system is not in deceleration fuel cut-off (DFCO) (4.8L only).

Conditions for Setting the DTC
    • The change in the MAP is less than a predetermined value during the EGR flow test.
    • All conditions are present for 2 seconds.

Action Taken When the DTC Sets
    • The control module illuminates the malfunction indicator lamp (MIL) when the diagnostic runs and fails.
    • The control module records the operating conditions at the time the diagnostic fails. The control module stores this information in the Freeze Frame/Failure Records.
    • The PCM disables the EGR valve for the ignition cycle.

Conditions for Clearing the MIL/DTC
    • The control module turns OFF the malfunction indicator lamp (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.

Diagnostic Aids

Important: Remove any debris from the PCM connector surfaces before servicing the PCM. Inspect the PCM connector gaskets when diagnosing or replacing the PCM. Ensure that the gaskets are installed correctly. The gaskets prevent water intrusion into the PCM.

Use a scan tool in order to clear information. Run the EGR flow test in order to verify a repair. Keep the following items in mind when running the EGR flow test:

    • The PCM runs the EGR flow test during a gradual deceleration.
    • The PCM runs the EGR flow test during a closed throttle condition.
    • The PCM runs the EGR flow test at vehicle speeds between 42-113 km/h (26-70 mph).
    • Several deceleration cycles may be necessary to run a sufficient number of EGR flow test samples (9-13).

Inspect for the following conditions:

    • Vacuum restriction to the MAP sensor--A restriction to the MAP sensor can cause the EGR flow test to fail due to insufficient MAP changes being monitored during the test. Inspect for objects blocking the vacuum to the MAP sensor.
    • An engine that is running poorly may cause this DTC to set.
    • 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 connectors and wiring harnesses related to the EGR valve. A change in the display indicates the location of the malfunction.

Reviewing the Failure Records vehicle mileage since the diagnostic test last failed helps determine when the condition that caused the DTC to be set occurred. This may assist in diagnosing the condition.

Test Description

The number below refers to the step number on the diagnostic table.

  1. This DTC is set when the PCM does not detect a predetermined change in MAP when the EGR valve is commanded. This step checks for any MAP sensor DTCs. A malfunctioning MAP sensor may cause this DTC to set.

DTC P0401 - EGR Flow Insufficient Detected

Step

Action

Yes

No

Schematic Reference: Engine Controls Schematics

1

Did you perform the Diagnostic System Check-Engine Controls?

Go to Step 2

Go to Diagnostic System Check - Engine Controls

2

Are there any MAP DTCs set?

Go to the applicable DTC table

Go to Step 3

3
  1. Inspect the exhaust system for modification of original installed parts or leaks.
  2. Repair the exhaust system as necessary if you locate a problem.

Did you find and correct the condition?

Go to Step 7

Go to Step 4

4
  1. Check the EGR valve gasket and pipes for leaks.
  2. If you locate a problem, clean or replace the EGR system components as necessary. Refer to Exhaust Gas Recirculation (EGR) System Cleaning .

Did you find and correct the condition?

Go to Step 7

Go to Step 5

5
  1. Remove the EGR valve. Refer to Exhaust Gas Recirculation Valve Replacement .
  2. Visually and physically inspect the following items for a leak or a restriction:
    3. • The EGR pintle
    • The EGR valve passages

Did you find and correct the condition?

Go to Step 7

Go to Step 6

6
  1. Remove the EGR inlet pipe from the exhaust manifold. Refer to Exhaust Gas Recirculation Pipe Replacement .
  2. Inspect the manifold EGR ports and the EGR inlet pipe for blockage caused by excessive deposits or other damage.
  3. Correct the condition as necessary if a problem is found.

Did you find and correct the condition?

Go to Step 7

Go to Diagnostic Aids

7
  1. Select the Diagnostic Trouble Code (DTC) option and the Clear DTC option using the scan tool.
  2. Idle the engine at the normal operating temperature.
  3. Select the Diagnostic Trouble Code (DTC) option and the Specific DTC option, then enter the DTC number using the scan tool.
  4. Operate the vehicle within the Conditions for Running this DTC as specified in the supporting text, if applicable.

Does the scan tool indicate that this test failed?

Go to Step 8

Go to Step 2

8

Select the Capture Info option and the Review Info option using the scan tool.

Does the scan tool display any DTCs that you have not diagnosed?

Go to the applicable DTC table

System OK