GM Service Manual Online
For 1990-2009 cars only

Circuit Description

The engine control module (ECM) performs a flow test on the exhaust gas recirculation (EGR) system during deceleration. The ECM does this by 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. If the ECM detects an EGR flow error, DTC P0401 sets.

DTC Descriptor

This diagnostic procedure supports the following DTC:

DTC P0401 Exhaust Gas Recirculation (EGR) Flow Insufficient

Conditions for Running the DTC

    • DTCs P0106, P0107, P0108, P0112, P0113, P0115, P0117, P0118, P0122, P0123, P0125, P0128, P0201, P0202, P0203, P0204, P0217, P0351, P0352, P0402, P0404, P0405, P0406, P0441, P0488, P0502, P0506, and P0507 are not set.
    • The EGR flow test is performed once per ignition cycle.
    • DTC P0401 runs multiple times on the first ignition cycle after the DTC is cleared from the ECM memory.
    • 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 ambient air temperature is more than 3°C (37°F).
       - The engine speed is between 1,550-2,900 RPM for automatic transmission or 1400-2,900 RPM for manual transmission before deceleration.
       - The manifold absolute pressure (MAP) is between 10.3-37 kPa.
       - The barometric pressure (BARO) is more than 72 kPa.
       - The vehicle speed is more than 18 km/h (11 mph) before deceleration.
    • The EGR flow test is ran when the following conditions are met during DFCO mode:
       - The throttle position is less than 1 percent.
       - The EGR position is less than 1 percent.
       - The MAP does not vary more than 1 kPa.
    • DTC P0401 runs continuously once the above conditions are met.

The vehicle speed needs to be more than 18 km/h (11 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. 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. The EGR flow test will be disabled if one of the following conditions occur:

    • The vehicle speed drops more than 4 km/h (3 mph).
    • The engine speed increases more than 50 RPM during a decel.
    • The actual EGR position is less than 90 percent of the commanded state.

Conditions for Setting the DTC

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

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.

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.

Test Description

The numbers below refer to the step numbers on the diagnostic table.

  1. MAP sensor faults must be diagnosed first. A skewed MAP sensor reading could cause this DTC to set.

  2. An engine mechanical condition may cause the engine to operate poorly, which could cause a low vacuum condition.

Step

Action

Yes

No

Schematic Reference: Engine Controls Schematics

Connector End View Reference: Engine Controls Connector End Views or Engine Control Module Connector End Views

1

Did you perform the Diagnostic System Check - Engine Controls?

Go to Step 2

Go to Diagnostic System Check - Engine Controls

2

  1. Observe the Freeze Frame/Failure Records for this DTC.
  2. Turn OFF the ignition for 30 seconds.
  3. Start the engine.
  4. 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.

Did the DTC fail this ignition?

Go to Step 3

Go to Intermittent Conditions

3

Is DTC P0106, P0107, or P0108 also set?

Go to Diagnostic Trouble Code (DTC) List

Go to Step 4

4

Inspect the exhaust gas recirculation (EGR) system for the following conditions:

    • 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

Refer to Exhaust Gas Recirculation Valve Replacement and Exhaust Gas Recirculation Valve Pipe Replacement .

Did you find and correct the condition?

Go to Step 9

Go to Step 5

5

Inspect for leaks or restrictions at the manifold absolute pressure (MAP) sensor.

Did you find and correct the condition?

Go to Step 9

Go to Step 6

6

Inspect the exhaust system for the following conditions:

    • Leaks caused by damage to the exhaust components
    • Restrictions that can cause excessive back pressure and low engine vacuum--Restrictions may be caused by aftermarket equipment or damage to the exhaust components.
    • Modification of original equipment manufacture (OEM) parts

Did you find and correct the condition?

Go to Step 9

Go to Step 7

7

  1. Inspect for the following engine mechanical conditions:
  2. • The correct installation of the timing belt
    • Worn piston rings
    • Worn camshaft
    • Other worn or damaged engine components
  3. Refer to Symptoms - Engine Mechanical in Engine Mechanical - 1.6L (L91).

Did you find and correct the condition?

Go to Step 9

Go to Step 8

8

Replace the EGR valve. Refer to Exhaust Gas Recirculation Valve Replacement .

Did you complete the replacement?

Go to Step 9

--

9

  1. Clear the DTCs with a scan tool.
  2. Turn OFF the ignition for 30 seconds.
  3. Start the engine.
  4. 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.

Did the DTC fail this ignition?

Go to Step 2

Go to Step 10

10

Observe the Capture Info with a scan tool.

Are there any DTCs that have not been diagnosed?

Go to Diagnostic Trouble Code (DTC) List

System OK