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
The following conditions will set the DTC:
| • | No active TP DTCs |
| • | No active MAP DTCs |
| • | No active VS DTCs |
| • | No active IAT DTCs |
| • | No active ECT DTCs |
| • | No active IAC DTCs |
| • | No active misfire DTCs |
| • | No active EGR pintle position DTCs |
| • | The ECT is more than 70°C (158°F) |
| • | The BARO is more than 70 kPa |
| • | The vehicle speed is more than 27 mph (43 km/h) |
| • | The change in IAC is less than 8 counts |
| • | The AC clutch status is unchanged |
| • | The transmission status is unchanged |
| • | The throttle position is less than 1 percent |
| • | The EGR position is less than 1 percent |
| • | The engine speed is between 900-1900 RPM (manual transmission) |
| • | The engine speed is between 800-1600 RPM (automatic transmission) |
| • | Any change in MAP is less than 0.5 kPa |
| • | DFCO not active (automatic transmission) |
| • | No change in DFCO (manual transmission) |
Conditions for Setting the DTC
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 check any circuitry that is suspected of causing the intermittent complaint. Refer to Intermittents and Poor Connections Diagnosis 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:
-
Correct any MAP sensor DTCs first.
-
This test ensures that the EGR valve is allowing EGR flow and the VCM is capable of controlling the EGR valve.
-
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 | Value(s) | Yes | No |
|---|---|---|---|---|
1 |
Important: Before clearing the DTCs, use the scan tool Capture Info to save the Freeze Frame and Failure Records for reference. The control module's data is deleted once the Clear Info function is used. Did you perform the Powertrain On-Board Diagnostic (OBD) System Check? | -- | ||
Did you find any MAP sensor DTCs? | -- | Go to the applicable DTC table | ||
Does the engine run rough as the EGR valve is cycled? | 1500 RPM | |||
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, tube or passages. Did you find a problem? | -- | Go to Diagnostic Aids | |
6 | Clean the EGR valve and passages. Is the action complete? | -- | -- | |
Does the scan tool indicate the diagnostic Passed? | -- | |||
8 | Does the scan tool display any additional undiagnosed DTCs? | -- | Go to the applicable DTC table | System OK |
