2000 S-Type 3.0 V6 with 205,000 miles
I'm getting OBD code P0401 Exhaust Gas Recirculation Insufficient Flow.
I reset it, and it comes back in 50 to 100 miles.

So far I have:
inspected all vacuum hoses, found no deficiencies;
checked electrical signal at electrically operated EGR vacuum regulator solenoid valve located on the LH side of the intake manifold;
checked fuse #6, replaced relay #14 which supplies power to EGR solenoid valve;
checked vacuum on input side of this valve (17.5'', 44 cm Hg), and at vacuum operated EGR valve connecting the EGR pipe to the intake manifold (1'', 4 cm);
cleaned vacuum operated EGR valve;
replaced EGR vacuum regulator solenoid valve;
checked pressure at both Differential Pressure Feedback Electronic DPFE sensor hoses (fluctuates about 1 psi 0.5 kg/scm);
replaced DPFE sensor located on the back of the intake manifold;
replaced vacuum operated EGR valve;
bypassed regulator solenoid valve with jumper hose and tried running for a few days with EGR valve wide open, thought it might blow out the EGR pipe;
removed 5/8'' diameter EGR pipe from exhaust manifold,
learned it has a 17/64'' (7mm) diameter orifice inside between the two small DPFE pipes, blew it out both directions including the small pipes with sandblaster using about 2 pounds (1 kg) of sand, used wire bottle brush to clean inside port on exhaust manifold;

Still getting code P0401. No other codes, car runs perfect.

What have I missed? What's left? Does anybody have any ideas?

 

With your high miles I would change the EGR valve. It may be working but you can't really tell.

No holes in the EGR tube?
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I did change the EGR valve already, step 9 above.
EGR tube is about 5 years old, no holes.

 

You either have the pipe carboned up or the intake carboned up.

 

Thanks. Step 11 was I sandblasted through the pipe, 100 psi with plenty of sand both directions, seems to have plenty of flow through. The pipe has only been on there about 50,000 miles. (I saved the old one which had a broken exhaust end, that's how I learned about the orifice, cut it open.) I also wire bottle brushed the ports on the intake and exhaust manifolds. Are you saying to take off the intake manifold and clean it?

 

Sorry I missed you replaced the EGR valve already.

Not sure as you have done a bunch of work and still have the code.

If you do get it fixed please let us know because it does not look like a normal EGR failure and maybe related to something else but still triggers the EGR code. Just guessing?
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I think I may have a clue to the problem. Bad electrical connection at the PCM. I tried to trace the wires from the DPFE back to the PCM. They are under the cabin air filter duct. You have to take off the windshield wipers, the plastic panel covering the wiper mechanism, the cabin air filter, and two plastic ducts, and there are three electrical connectors under there. I couldn't get them disconnected, but I wiggled them. After that the engine light stayed off for over a week. It came on again with the same code yesterday. So I think I need to take the PCM out from the inside and clean the contacts.

 

Hey, I have the same problem with another code p0420,
my question is,
Would it be safe and okay to drive my car for 500 mile trip without fixing this codes??

 

To confirm you changed out both the valve and vacuum solenoid? Why did you have to change the relay?

You might need to clean out the passages in the manifold itself.