An exhaust gas recirculation (EGR) system is used to lower nitrogen oxide (NOx) emission levels caused by high combustion temperatures. It 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 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 an engine control module (ECM) controlled pintle. The ECM controls the pintle position using inputs from the throttle position (TP) and manifold absolute pressure (MAP) sensors. The ECM then commands the EGR valve to operate when necessary by controlling an ignition signal through the ECM. This can be monitored on a scan tool as the Desired EGR Position.

The ECM monitors the results of its command through a feedback signal. By sending a 5-volt reference and a ground to the EGR valve, a voltage signal representing the EGR valve pintle position is sent to the ECM. This feedback signal can also be monitored on a scan tool and is the actual position of the EGR pintle. The Actual EGR Position should always be near the commanded or Desired EGR Position.

This diagnostic will determine if there is a reduction in EGR flow.

This diagnostic procedure supports the following DTC:

DTC P0401 Exhaust Gas Recirculation (EGR) Flow Insufficient

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

The EGR Decel Filter value can be a great aid in determining if a problem exists and to verify repairs. The EGR Decel Filter is an average of the difference in the expected MAP change and the actual MAP change caused by opening the EGR valve during a deceleration, and is used to determine when the MIL is illuminated. By driving the vehicle up to approximately 97 km/h (60 mph) and decelerating to 32 km/h (20 mph), it can be determined if the EGR system is OK, partially restricted, or fully restricted.

A more negative number, less than -3, indicates that the system is working normally, whereas a positive number indicates that the system is being restricted and that the expected amount of EGR flow was not seen. A number that falls between -3 and +2 indicates that the system is partially restricted, but not restricted enough to cause an emissions impact.

The EGR Decel Filter value should always be at -3 or lower. If the EGR Decel Filter number becomes more positive, towards 0 or more, then the EGR system is becoming restricted. Look for possible damage to the EGR pipe or for a restriction caused by carbon deposits in the EGR passages or on the EGR valve.

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

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

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