P0401
DTC P0401: Exhaust Gas Recirculation (EGR) Flow InsufficientCircuit Description
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.
Conditions for Setting the DTC
- IAT is between 2.75 and 80 °C.
- Vehicle speed is greater than 18 km/h.
- Throttle position is less than 1.001%.
- Barometric Pressure (BARO) is greater than 70 kPa.
- Engine speed is greater than 1,200 rpm.
(CASE A)
- EGR position is less than 1.001%.
- Compensated Manifold Absolute Pressure (MAP) is between 10 kPa and 32 kPa.
- Engine speed is between 1,300 rpm and 2,900 rpm.
- Delta MAP is less than 1 kPa.
(CASE B)
- DFCO status condition met.
- Delta idle airflow is less than 15 g/s.
Action Taken When the DTC Sets
- The Malfunction Indicator Lamp (MIL) will illuminate.
- The ECM will record operating conditions at the time the diagnostic fails. This information will be stored in the Freeze Frame and Failure Records buffers.
- Low speed coolant fan turns ON.
- A history DTC is stored.
Conditions for Clearing the MIL/DTC
- The MIL will turn OFF after four consecutive ignition cycles in which the diagnostic runs without a fault.
- A history DTC will clear after 40 consecutive warm up cycles without a fault.
- The DTC(s) can be cleared by using the scan tool.
- Disconnecting the ECM battery feed for more than 10 seconds.
Diagnostic Aids
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 is was not seen. A number that falls between negative 3 and positive 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.
Test Description
Number(s) below refer to the step number(s) on the Diagnostic Table.
1. The On-Board Diagnostic (OBD II) System Check prompts the technician to complete some basic checks and to store the Freeze Frame and Failure Records data on the scan tool if applicable. This creates an electronic copy of the data taken when the malfunction occurred.
The information is then stored on the scan tool for later reference.
2. Commanding the EGR valve open determines whether the EGR system is totally restricted or partially restricted.
3. Visually and physically inspect the EGR passages and valve for excessive carbon deposits or damage.
4. Be sure all gasket material is removed from the EGR mounting surface. Even a small amount of material may cause a DTC P0401 to set.
5. This step verifies if the fault is present and also verifies if a repair corrected the problem. If the EGR Deceleration Filter value stays near 0 or a positive number after several tests have been run, then a small restriction may still exist. Be sure to check the EGR pipe for damage or dents and the EGR valve for any excessive carbon build up. Only 1 test per ignition cycle will run unless a DTC P0401 has been cleared or the battery has been disconnected.
6. Cleaning DTCs is a very important step for this diagnostic. The cleaning function allows the EGR valve to relearn a new pintle position as the old pintle position was inaccurate due to the failure that caused the DTC. The DTC must be cleared with the ignition ON, engine OFF or when the engine is idling. If the ECM sees a EGR command, the new pintle will not learned.
Step 1-7:
Troubleshooting