P0401 is almost always the DPFE starting to fail, looks can be deceiving.
Here is how the EGR system works so you can eliminate other possibilities before replacing the DPFE.
There are 3 parts for the EGR system, not counting the computer(PCM)
1. EGR valve and tube, this valve is opened by applying vacuum to its port
2. EGR modulator(solenoid or activator), it is a 12volt vacuum valve, it passes vacuum to the EGR valve, PCM controls this by sending it 12volts, it is often a pulsed 12volts, not simply an on/off, this allows PCM to open the EGR valve a little or alot.
3. DPFE sensor measures the pressure difference in the exhaust manifold, it has 2 hoses, one is near or on the EGR tube, the other is on the exhaust manifold.
When PCM opens the EGR valve the pressure in the hose on/near the EGR valve/tube will drop, how much it drops tells PCM now far open the valve is, so how much exhaust gas is being added to intake.
P0401: Insufficient EGR Flow Detected
This code comes from the DPFE sensor telling the PCM that there is no or little pressure change after PCM has started to open EGR valve.
It could be the EGR valve is not opening, so leaky vacuum hose back to EGR modulator or valve is broken.
Get a 3ft piece of vacuum hose that will fit EGR Valve port, install it on the EGR valve, start engine, suck on the end of the hose, create a vacuum, EGR valve should start to open and engine should start to run rough and even die, EGR valve is working.
Follow the EGR valve's vacuum hose back to the EGR modulator, remove it and test it for leaks, i.e. cracks or holes.
reinstall if it is good.
Remove the other vacuum hose from the EGR modulator, and follow it to the intake manifold, remove it and check for leaks and cracks.
reinstall if good.
EGR modulators rarely fail, not never it just isn't common, you can remove the connector and apply 12volt or even use a 9volt battery to test if it "clicks", also put that 3ft vacuum hose on it and suck on the end while activating the Modulator, it should release "your vacuum" when activated.
Now the DPFE sensor.
Where the two hoses go matters, so remove only 1 at a time, they are usually different sizes but not always, check each hose for leaks, they are on the hot exhaust pipes so can get brittle and crack on that end.
If both hoses are OK then time to test DPFE with Voltage meter.
DPFE will have 3 wires
Ground
5volts from PCM
Voltage out to PCM(varies from 0-5volts)
You will need a Volt meter and a sewing pin or safety pin to pierce the wire to test voltage.
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