The 402 code results from a leak in the pipe to intake tract thru the EGR valve at some point.
Just before starting the motor, the PCM looks at the DPFE for a reference reading as the basis for the test of system tighness after the motor is started. (exhaust pressure forced into the closed system upon cold start acceleration).
If the DPFE outputs a signal indicating there is a flow of any kind ('lower' than expected pressure) such as thru the EGR seat, a leaking tube, leaking DPFE atttaching tubes or the vacuum gate valve is leaking letting the EGR move off it's seat, or the EGR seat is leaking for some reason, you will see the 402 code test failure.
As you see, there is cleaver engineering put into these PCM systems to allow testing and reporting of the problems.
Any failure means the whole system will not pass PCM diagnostic self testing and operate as designed.
This is modern system control and will not get any better very soon.
Other makes have the same types of issues.
The systems are designed so you cannot 'legally' get around them without being detected.
.
BTW, systems have already been designed to take the whole thing out of your hands if the gov. forces the issue in the future.
What that means is there are a number of ways to remote monitor vehiches such that you would get a notice in the mail that your vehichle is non compliant, have it fixed and report; and you would not even know when or where it was checked. All done by two way radio telemetry.
It's on the shelf waiting for the day it can be forced on all new vehichles sometime in the future.
Said another way, emmissions no longer has to be checked at the tail pipe because the PCM and hardware are that good at self checking and reporting. It's just a matter of addressing the PCM by signaling the onboard reciever and requesting it to output data to an onboard transmitter, complete with ID and all.
Merry Christmas.

 

GM has already been using it for years. It's called On Star by GM, but the government calls it OBDIII. I think Ford is beginning to use it based upon some recent TV commercials I've seen.

 

Changed all the parts and kept getting code po401. I finally cleaned the 2 tubes on the exaust coming off the dpfe hoses. I used a small piece of copper and reamed out the openings and put hoses back on.

 

my 98 f150 is missing the tail pipe right after the catylitic converter...could this cause a p0401 code...

 

Lorne, the 401 code is a 'lack' of flow in the EGR system when tested by the diagnostic program.
You have the cats on the driver side to offer enough back pressure that a missing muffler likely would not make a difference to that test since the exhaust pressure is taken right at the exhaust manifold.
Clear the blockage in the system or a faulty DPFE or both.
Don't replace the DPFE unless your sure there is no blockage between the EGR and the intake passage, and the vacuum line fro the intake to the EVR and EGR are inspected as good.
Good luck.

 

You need to know EVAP basics first. It's really a stupid system as you can't get far on fumes which is all EVAP sends to the engine. If it were to actually send liquid gasoline which is what the engine runs on you'd have a BIG EVAP problem. Ford has a problem with the passage in the intake clogging shut. Everything is working properly, but there is still NO FLOW. If you guys haven't figured out solenoids, relays, diaphrams and DPFE by now you never will. Buy an electric car and forget all this stuff. This thread is over 5 years old. CARQUEST(bought out by Advance Auto Parts) is teaching EVAP at one of the classes next year. I'll be there, but I wish this class had been taught 5 years ago. Contact CARQUEST Technical Institute and ask about instructor lead classes in your area. The cost is $1,200.00 per year per shop and that's a bargain. Up to 6 techs per shop in a year. I've been going for years and I finally know the names of all the factory scan tools and all the auto industry acronyms. Check it out.

 

here's the real story on the egr. the offending ports are not in the throttle body, they're in the plenum. this isn't speculation, it's how i spent yesterday. you're going to have to remove the plenum and clean them out. once it's off and in hand, you'll see the problem. mine were completely obstructed. and no, it can't be done in place.
you'll need:
egr gasket
iac gasket
throttle body gasket
manifold/plenum gasket

remove hoses, cables and unplucg wires
remove egr valve
remove iac
remove throttle body
remove the plenum
i had to use a drill bit held in visegrips to ream the passages. start small and work up. i ended at 13/32". spray gasket remover will soften hard carbon, just don't get it on you hands, it burns like all getout. the port from the valve i cleaned with a 20 guage shotgun brush on a slow drill.
once you get all the hard deposits out, clean and reassemble. you'll be GTG for however long it took to clog (240K in my case)

 

I am new to forums so I don't know if I'm looking right but I have a 401 code on my 2003 f-150 4.2l v-6. I am told there are egr ports to clean but my egr does not mount to the throttle body.it mounts to a separate manifold that feeds directly into the intake with a large diameter passage. I have already replaced the following, dpfe, egr, and plugs and wires. is there a diagram somewhere I can look at

 

As your first post, you should have started a new thread.
This thread is spaced out over a number of years.
You have a V6 and is equipped differently than a V8 motor so be careful what you read about.
Your motor has separate ports for each cylinder inside the intake manifold.
.
You replaced plugs and wire that have nothing to do with the EGR operation.
The 401 code is a low flow test failure when the computer does the testing for flow rate as detected by the DPFE 'sensor'.
Low flow is a blockage to that flow test as reported as a code 401.
To test for blockage from the EGR into the intake, remove the EGR connection.
Apply 'substitute vacuum' to open the EGR valve.
If the motor reacts with stumble and roughness it tells you that tract (is open and clean) such that the problem is before the EGR causing the code to set.
If you have changed the DPFE that eliminates it as a cause unless the new one is faulty where a Ford part should be used..
What is left is the exhaust feed to the EGR from the exhaust manifold and the Vacuum solenoid that passes vacuum to open the EGR.
Is the solenoid (called the EVR) sticking closed etc.
Are the hoses and vacuum line involved in good condition?
Don't look anywhere else for the problem.
Good luck.