The exhaust gas recirculation (EGR) valve is controlled by the engine control module (ECM). The ECM controls the EGR valve by suppling a 12-volt pulse width modulated (PWM) signal to the EGR solenoid high control circuit and a ground to the low control circuit. The ECM monitors the EGR flow efficiency when the EGR is active by comparing the desired Mass Air Flow (MAF) to the actual MAF. If the actual MAF is less than or greater than a calibrated threshold level the amount of detected EGR flow is out of range.

P0401

The ECM detects that the difference between the actual mass air flow and the desired mass air flow is less than a calibrated value for greater than 15 seconds when the EGR is active.

P0402

The ECM detects that the difference between the actual mass air flow and the desired mass air flow is greater than a calibrated value for greater than 15 seconds when the EGR is active.

P0401

DTC P0401 is a Type B DTC.

P0402

DTCs P0401 and P0402 are Type B DTCs.

Inspect for the following conditions that may cause this DTC to set:

Schematic Reference

Engine Controls Schematics

Connector End View Reference

Component Connector End Views

DTC Type Reference

Powertrain Diagnostic Trouble Code (DTC) Type Definitions

Scan Tool Reference

Control Module References for scan tool information

1. Verify if DTC P003A, P0101, P0403, P0405, P0406, P046C, P0698, or P0699 are not set.

⇒	If any of the DTCs are set, refer to Diagnostic Trouble Code (DTC) List - Vehicle.

2. Ignition ON, observe the Desired EGR Position and the EGR Position Sensor parameters. With a scan tool, command the EGR solenoid to 20 percent in 5 percent increments. The Desired EGR Position and the EGR Position Sensor parameters should respond and remain within 3  percent of each other with each command.

⇒	If greater than the specified range or the EGR Position Sensor parameter does not respond and remains stationary, replace the EGR valve and gaskets.

3. Engine running at idle speed, perform the following:

•	With a scan tool, command the EGR solenoid to 25 percent.

•	Increase the engine speed to 2,500 RPM. The engine should run rough.

4. Operate the vehicle within the Conditions for Running the DTC to verify that the DTC does not reset. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records data.

Important: 

1. Engine running at idle, ECT parameter must be greater than 80°C (176°F). Perform the following:

1.1.	Maintain engine speed at idle.

1.2.	With a scan tool, command the EGR solenoid to 0 percent and record the Mass Air Flow g/s parameter.

1.3.	With a scan tool, command the EGR solenoid to 40 percent while observing the actual EGR Position and the Mass Air Flow parameters. Record the Mass Air Flow g/s parameter. The Mass Air Flow g/s parameter should decrease by 15 g/s or more.

⇒	If the Mass Air Flow parameter does not decrease by 15 g/s or more, inspect the EGR cooler and the EGR valve passages for a restriction.

2. Ignition OFF, inspect for the following:

•	The exhaust system for leaks, restrictions, or for modification of the original equipment manufacture (OEM) part

•	For a restriction in the air cleaner and the air intake ducts.

•	For a restriction in the turbocharger charge air cooler (CAC) system.

•	For a restriction in the intake manifold.

3. Test the air induction system for air leaks between the MAF sensor and the turbocharger. Perform the Induction System Smoke Test. Refer to Charge Air Cooler Diagnosis.
4. Test the charge air cooler system for leaks. Perform the Full System Air Leak Test. Refer to Charge Air Cooler Diagnosis.
5. If the air induction, charge air cooler (CAC), and mass air flow test normal, operate the vehicle within the Conditions for Running the DTC. If the DTC resets, perform the Component Testing.

Important: Perform the Circuit/System Testing before proceeding with the Component Testing.

1. Remove the EGR valve. Refer to Exhaust Gas Recirculation Valve Replacement.
2. Inspect the EGR valve, the EGR cooler, the air inlet tube, and all of the EGR passages for the following conditions:

•	A sticking, binding, or frozen EGR valve

•	A leak or restriction in the EGR valve and EGR ports and passages

•	Any coking, soot build up, debris, damage, that can cause the EGR valve to stick partially open or closed

⇒	If any of the above conditions are found, replace the affected component and gaskets, as necessary.

1. Install any components or connectors that have been removed or replaced during diagnosis.
2. Perform any adjustment, programming, or setup procedures that are required when a component or module is removed or replaced.
3. Clear the DTCs.
4. Turn OFF the ignition for 60 seconds.
5. Duplicate the Conditions for Running the DTC and use the Freeze Frame/Failure Records, if applicable, in order to verify the DTC does not reset. If the DTC resets or another DTC is present, refer to Diagnostic Trouble Code (DTC) List - Vehicle and perform the appropriate diagnostic procedure.
6. Perform the Diesel Particulate Filter (DPF) Service Regeneration.