Hi here is a list of possible failures, but usually it's a bad egr valve(99%). Ours(factory) come complete with solenoid. I am also attaching some diagnostics too. Let me know if you need anything else.
P0401 - EGR SYSTEM FAILURE
When Monitored: With the engine temperature greater than 82° C (180° F), and the
ambient temperature above 32° C (89° F), the engine is in closed loop. The O2 sensor is
operational, and the engine is under a steady load with the throttle open, and the 02
controller is not at or near its limits.
Set Condition: Conditions are met, the EGR is turned off and on momentarily. The
oxygen sensor signal is monitor for changes in its output signal during this test. If no
significant change or too much change is seen, a flag is set.
EGR SOLENOID VACUUM SUPPLY
EGR VALVE OPERATION (CLOSED)
EGR VALVE OPERATION (OPEN)
FUSED IGNITION SWITCH OUTPUT CIRCUIT OPEN
EGR SOLENOID CONTROL CIRCUIT SHORTED TO VOLTAGE
EGR SOLENOID CONTROL CIRCUIT OPEN
EGR SOLENOID (CLOSED)
EGR SOLENOID CONTROL CIRCUIT SHORT TO GND
EGR SOLENOID (OPEN)
TEST ACTION APPLICABILITY
1 Turn the ignition on.
With the DRBIIIt, read DTC's.
Is the Good Trip displayed and equal to zero?
Yes ! Go To 2
No ! Go To 12
2 Start the engine.
Allow the engine to reach operating temperature.
Does the engine run rough or stall at idle?
Yes ! Go To 3
No ! Go To 6
3 Disconnect the vacuum hose from the EGR Valve.
Start the engine and allow it to idle.
Yes ! Remove the EGR Valve and inspect the valve for an obstruction
that would keep the valve open. If OK, replace the EGR Valve.
Perform POWERTRAIN VERIFICATION TEST VER - 5.
No ! Go To 4
4 Turn the ignition off.
Disconnect the PCM harness connector.
Disconnect the EGR Solenoid harness connector.
Measure the resistance of the EGR Solenoid Control circuit in the EGR Solenoid
harness connector to ground.
Is the resistance below 5.0 ohms?
Yes ! Repair the EGR Solenoid Control circuirt for a short to ground.
No ! Go To 5
5 Turn the ignition off.
Using a 12-volt test light connected to 12-volts, probe the EGR Solenoid Control
Turn the ignition on.
With the DRBIIIt, actuate the EGR Solenoid.
Observe the test light.
Does the test light flash on and off?
Yes ! Replace the EGR Solenoid.
No ! Replace and program the Powertrain Control Module in accordance
with the Service Information.
6 Turn the ignition off.
Disconnect the EGR Solenoid vacuum supply at the EGR Solenoid.
Connect a vacuum gauge to the EGR Solenoid vacuum supply.
Start the engine.
Observe the vacuum gauge.
Does the vacuum gauge read above 109?
Yes ! Go To 7
No ! Repair the EGR Solenoid vacuum supply.
7 Disconnect the vacuum hose from the EGR Valve. Inspect the vacuum hose for any
Connect a Vacuum Pump to the EGR Valve.
Using the Vacuum Pump, apply vacuum to the EGR Valve.
Yes ! Go To 8
No ! Remove the EGR Valve and inspect the valve for an obstruction
that would keep the valve from opening. If OK, replace the EGR
8 Turn the igntion off.
Using a 12 volt Test Light connected to ground, probe the Fused Ignition Switch
Output circuit in the EGR Solenoid harness connector.
Does the 12 volt test light illuminate?
Yes ! Go To 9
No ! Repair the Fused Ignition Switch Output circuit for an open.
9 Turn the ignition off.
Measure the voltage of the EGR Solenoid Control circuit in the EGR Solenoid
Is the voltage above 1.0 volts?
Yes ! Repair the EGR Solenoid Control circuit for a short to voltage.
No ! Go To 10
10 Turn the ignition off.
Measure the resistance of the EGR Solenoid Control circuit between the PCM
harness connector and the EGR Solenoid harness connector.
Yes ! Go To 11
No ! Repair the EGR solenoid control circuit for an open.
11 Turn the ignition off.
circuit in the EGR Solenoid harness connector.
Does the test light flash on and off.
Yes ! Inspect the EGR Solenoid Back Pressure hose and tube for
obstructions or damage. If OK, replace the EGR Solenoid.
WARNING: KEEP CLEAR OF THE ENGINE'S MOVING PARTS.
NOTE: The conditions that set the DTC are not present at this time. The
following list may help in identifying the intermittent condition.
With the engine running at normal operating temperature, monitor the DRBIIIt
parameters related to the DTC while wiggling the wiring harness. Look for parameter
values to change and/or a DTC to set.
Review the DRBIIIt Freeze Frame information. If possible, try to duplicate the
conditions under which the DTC was set.
Refer to any Technical Service Bulletins (TSB) that may apply.
Visually inspect the related wiring harness. Look for any chafed, pierced, pinched, or
partially broken wires.
Visually inspect the related wiring harness connectors. Look for broken, bent, pushed
out, or corroded terminals.
Were any of the above conditions present?
Yes ! Repair as necessary
No ! Test Complete.