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 VCM tests the 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 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 TP sensor DTCs |
| • | No active MAP sensor DTCs |
| • | No active VS sensor DTCs |
| • | No active IAT sensor DTCs |
| • | No active ECT sensor DTCs |
| • | No active IAC sensor DTCs |
| • | No active misfire DTCs |
| • | No active EGR pintle position DTCs |
| • | The ECT is more than 50°C (122°F). |
| • | The BARO is more than 75 kPa. |
| • | The vehicle speed is more than 24 km/h (15 mph). |
| • | The change in IAC is less than 5 counts. |
| • | The AC clutch status is unchanged. |
| • | The transmission status is unchanged. |
| • | The engine has been running for more than 120 seconds. |
| • | The throttle position is between 4-30 percent. |
| • | The EGR position is more than 15 percent. |
| • | The engine speed is between 1200 RPM and 2200 RPM. |
| • | Any change in MAP is less than 0.4 kPa. |
Conditions for Setting the DTC
During the EGR flow test, the change in MAP is less 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
Intermittent test-- 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.
-
This test ensures that the EGR valve is allowing EGR flow and the VCM is capable of controlling the EGR valve.
-
This step checks the MAP sensors ability to correctly indicate barometric pressure.
In order to determine the correct MAP sensor display of barometric pressure for any elevation, perform either of the following:
-
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.
| • | Compare the recorded MAP sensor display value to the BARO display on a known good vehicle. |
| • | Obtain the local weather service barometer reading (in.Hg). Multiply the barometer reading by 3.4. The result indicates the correct BARO value for your area. |
Step | Action | Values | Yes | No |
|---|---|---|---|---|
1 | Did you perform the Powertrain On-Board Diagnostic (OBD) System Check? | -- | ||
Does the engine run rough as the EGR valve is cycled? | 1500 RPM | |||
Is the MAP value of the suspect vehicle within the specified value of the known good vehicle? | 3 kPa | |||
4 |
Did you find a problem? | -- | ||
5 | This DTC is intermittent. Are there any additional DTCs stored? | -- | Go to the applicable DTC table | Go to Diagnostic Aids |
6 |
Does the test lamp turn ON as the EGR valve is cycled? | -- | ||
7 | Inspect for a restriction in the EGR valve, tube or passages. Did you find a problem? | -- | ||
8 | Replace the EGR valve. Refer to Exhaust Gas Recirculation Valve Replacement . Is the action complete? | -- | -- | |
9 | Inspect for an open in the EGR valve ground circuit. Did you find a problem? | -- | ||
10 | Replace the MAP sensor. Refer to Manifold Absolute Pressure Sensor Replacement . Is the action complete? | -- | -- | |
11 | Clean the EGR valve and passages. Is the action complete? | -- | -- | |
12 | Inspect for an open or short in the EGR valve control circuit. Did you find a problem? | -- | ||
13 | Inspect for a poor connection at the VCM. Did you find a problem? | -- | ||
14 | Repair the circuit as necessary. Refer to Wiring Repairs in Wiring Systems. Is the action complete? | -- | -- | |
15 |
Important: The replacement VCM must be programmed. Replace the VCM. Refer to VCM Replacement/Programming . Is the action complete? | -- | -- | |
Does the scan tool indicate the diagnostic Passed? | -- | |||
17 | Does the scan tool display any additional undiagnosed DTCs? | -- | Go to the applicable DTC table | System OK |
