GM Service Manual Online
For 1990-2009 cars only

Object Number: 600569  Size: MF
Engine Controls Components
Emission Controls
OBD II Symbol Description Notice
Handling ESD Sensitive Parts Notice
Handling ESD Sensitive Parts Notice

Circuit Description

An exhaust gas recirculation (EGR) system is used in order to lower oxides of nitrogen (NOx) emission levels caused by high combustion temperatures. The EGR accomplishes this by feeding small amounts of exhaust gases back into the combustion chamber. When the air/fuel mixture is diluted with the exhaust gases, combustion temperatures are reduced.

A linear EGR valve is used on this system. The linear EGR valve is designed in order to accurately supply exhaust gases to the engine without the use of intake manifold vacuum. The valve controls exhaust flow going into the intake manifold from the exhaust manifold through an orifice with a control module controlled pintle valve. The control module commands the EGR by applying a 12 volt duty cycle to the EGR valve. This can be monitored on a scan tool as desired EGR position.

The control module can monitor the pintle position with the EGR pintle position signal. This sensor uses a variable resistor with a 5 volt reference, a signal circuit, and a ground. The signal circuit monitors changes in voltage on the variable resistor as the pintle moves.

The vehicle control module (VCM) tests the EGR system during deceleration by momentarily commanding the exhaust gas recirculation (EGR) valve open. The pressure in the intake manifold increases when the EGR valve is open. The VCM monitors the manifold absolute pressure (MAP) sensor signal during the EGR system diagnostic in order to determine the amount of EGR flow. The MAP sensor will detect an increase in manifold pressure as the EGR is cycled ON. If the amount of manifold pressure observed is less than expected by the VCM, a DTC P0401 will be set.

Conditions for Running the DTC

    • No active throttle position (TP) sensor DTCs
    • No active manifold absolute pressure (MAP) sensor DTCs
    • No active vehicle speed sensor (VSS) DTCs
    • No active intake air temperature (IAT) sensor DTCs
    • No active engine coolant temperature (ECT) sensor DTCs
    • No active idle air control (IAC) DTCs
    • No active misfire DTCs
    • No active EGR pintle position DTCs
    • The ECT is more than 70°C (158°F).
    • The barometric pressure (BARO) is more than 70 kPa.
    • The vehicle speed is more than 43 km/h (27 mph).
    • The change in IAC is more than 8 counts.
    • The AC clutch status is unchanged.
    • The transmission status is unchanged.
    • The throttle position is more than 1 percent.
    • The EGR position is more than 1 percent.
    • The engine speed is between 1,000-1,900 RPM, with a manual transmission.
    • The engine speed is between 800-1,600 RPM, with an automatic transmission.
    • Any change in MAP is more than 0.5 kPa.
    • The DFCO is not active, with an automatic transmission.
    • No change in DFCO status, with a manual transmission.

Conditions for Setting the DTC

The change in MAP is more than a calibrated value for more than 2 seconds.

Action Taken When the DTC Sets

    • The control module illuminates the malfunction indicator lamp (MIL) the first time the diagnostic runs and fails.
    • The control module will set the DTC and records the operating conditions at the time the diagnostic fails. The control module stores the failure information in the scan tools Freeze Frame/Failure Records.

Conditions for Clearing the MIL or DTC

    • The control module turns OFF the MIL after 3 consecutive drive trips when the test has run and passed.
    • A history DTC will clear if no fault conditions have been detected for 40 warm-up cycles. A warm-up cycle occurs when the coolant temperature has risen 22°C (40°F) from the startup coolant temperature and the engine coolant reaches a temperature that is more than 70°C (158°F) during the same ignition cycle.
    • Use a scan tool in order to clear the DTCs.

Diagnostic Aids

Notice: In order to prevent further damage if the EGR valve shows signs of excessive heat, check the exhaust system for blockage (possibly a plugged converter) using the procedure found on the restricted exhaust system check. If the exhaust system is restricted, repair the cause; one of which might be an injector which is open due to one of the following reasons:

   • Stuck
   • Grounded driver circuit
Check the oil for possible fuel contamination if a stuck open fuel injector is found.

Inspect for intermittents. If connections and harness check OK, monitor a digital voltmeter connected between terminal EGR control circuit and ground while moving related connectors and wiring harness. If the failure is induced, the voltage reading will change.

An intermittent may be caused by any of the following conditions:

    • A poor connection
    • Rubbed through wire insulation
    • A broken wire inside the insulation

Thoroughly inspect any circuitry that is suspected of causing the intermittent complaint. Refer to Testing for Intermittent Conditions and Poor Connections in Wiring Systems.

If a repair is necessary, refer to Wiring Repairs or Connector Repairs in Wiring Systems.

Test Description

The numbers below refer to the step numbers in the diagnostic table:

  1. Correct any MAP sensor DTCs first.

  2. This test ensures that the EGR valve is allowing EGR flow and the VCM is capable of controlling the EGR valve.

  3. Clearing the DTCs is a very important step in this diagnostic procedure. The clearing function allows the EGR valve to relearn a new minimum pintle position, as the old position was inaccurate due to the malfunction that caused this DTC.

Step

Action

Values

Yes

No

1

Did you perform the Powertrain On-Board Diagnostic (OBD) System Check?

--

Go to Step 2

Go to Powertrain On Board Diagnostic (OBD) System Check

2

Did you find any MAP sensor DTCs?

--

Go to the applicable DTC table

Go to Step 3

3

  1. Install a scan tool.
  2. Start the engine.
  3. Run the engine at more than 1,500 RPM.
  4. Cycle the EGR valve to 100  percent with a scan tool. The RPM should decrease as the EGR valve is cycled.

Does the engine run rough as the EGR valve is cycled?

--

Go to Step 4

Go to Step 5

4

This DTC is intermittent.

Are there any additional DTCs stored?

--

Go to the applicable DTC table

Go to Diagnostic Aids

5

Check for a restriction in the EGR valve, the tube, or the passages.

Did you find a problem?

--

Go to Step 6

Go to Diagnostic Aids

6

Clean the EGR valve and the passages.

Is the action complete?

--

Go to Step 7

--

7

  1. Using the scan tool, clear the DTCs.
  2. Start the engine.
  3. Allow the engine to idle until the engine reaches a normal operating temperature.
  4. Select the DTC and the Specific DTC functions.
  5. Enter the DTC number which was set.
  6. Operate the vehicle within the Conditions for Setting the DTC until the scan tool indicates the diagnostic Ran.

Does the scan tool indicate the diagnostic Passed?

--

Go to Step 8

Go to Step 2

8

Does the scan tool display any additional undiagnosed DTCs?

--

Go to the applicable DTC table

System OK