GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Chevrolet 🢒 2001 🢒 Tahoe - 4WD 🢒 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 a DTC P0401 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. In order to aid in verifying a repair, the PCM allows more than one sample per ignition cycle until 13 samples have been obtained after a scan tool Clear Info or a battery disconnect.

Conditions for Running the DTC
    • DTCs P0101, P0102, P0103, P0106, P0107, P0108, P0112, P0113, P0117, P0118, P0300, P0404, P0405, P0500, P0502, P0503, P0506, P0507, P0562, P0563, P1120, P1125, P1220, or P1221 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 60-118°C (140-244°F).
    • The intake air temperature (IAT) is between 3-75°C (37-167°F).
    • The engine speed is between 725-2,000 RPM.
    • The throttle position (TP) sensor angle is less than 1.2 percent.
    • The altitude compensated MAP is between 10-60 kPa (3-6 psi).
    • The vehicle speed is between 40-113 km/h (25-70 mph).
    • The change in vehicle speed sensor (VSS) is less 6 km/h (4 mph).
    • The change in MAP is less than 0.5 kPa.
    • The A/C clutch status is steady.
    • The fuel system is not in deceleration fuel cut-off.

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

Action Taken When the DTC Sets
    • The PCM will illuminate the malfunction indicator lamp (MIL) during the first trip in which the diagnostic test has been run and failed.
    • The PCM will store conditions which were present when the DTC set as Freeze Frame/Failure Records data.
    • 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

Use a scan tool in order to clear info. 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 40-113 km/h (25-70 mph).
    • Several deceleration cycles, typically 9-13, may be necessary in order to run a sufficient number of EGR flow test samples.

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.

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 Diagnostic Trouble Code (DTC) List

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 and/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 pipe from the exhaust manifold and the intake manifold. Refer to Exhaust Gas Recirculation Pipe Replacement .
  2. Inspect the manifold EGR ports and the EGR pipe for a blockage caused by excessive deposits or other damage.

Did you find and correct the condition?

Go to Step 7

Go to Diagnostic Aids

7
  1. Use the scan tool in order to clear the DTCs.
  2. Turn OFF the ignition for 30 seconds.
  3. Start the engine.
  4. Operate the vehicle within the Conditions for Running the DTC as specified in the supporting text.

Does the DTC run and pass ?

Go to Step 8

Go to Step 2

8

With a scan tool, observe the stored information, Capture Info.

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

Go to Diagnostic Trouble Code (DTC) List

System OK