Help Please P0401
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Freshman User
Joined: Nov 2008
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Help Please P0401
I have a 96 5.0L 120,000 miles with CEL code P0401 EGR is good and line good to new EGR Vacuum Regulator. I changed the DPFE vavle (used) and still have light come on after I reset codes. What should I ck next? I posted on the older truck Thead but people don't respond to 96 models 5.0L much!!! Thanks for any info!!! I have got to get this light off to get a state sticker!!!! 
Lead Driver
Joined: Sep 2006
Posts: 7,907
Likes: 143
401 code is a low flow test failure.
Questions.
Is the EGR port blocked behind the EGR valve?
How do you know thw EGR is good?
How do you know the used DPFE is good?
Is the tube from the exhaust manifold open and clear?
Is the EVR operational and hoses tight?
What I am saying is the flow test the computer does depends on a calibrated flow rate as detected by the DPFE.
You can see the questions address the flow path and the DPFE has to be good to detect the correct flow rate and report it back to the computer.
If at the end of the 'SECOND" test, the cpmputer does not see the correct reported value, the P0401 code is set and your CEL comes on.
Once the code is cleared, the test is done two times before a failure is declared and a code is set so it will fool you if the drive cycle is not done twice.
Do the diagnostic work correctly, use known good Ford parts and the issue will be resolved.
Good luck.
Questions.
Is the EGR port blocked behind the EGR valve?
How do you know thw EGR is good?
How do you know the used DPFE is good?
Is the tube from the exhaust manifold open and clear?
Is the EVR operational and hoses tight?
What I am saying is the flow test the computer does depends on a calibrated flow rate as detected by the DPFE.
You can see the questions address the flow path and the DPFE has to be good to detect the correct flow rate and report it back to the computer.
If at the end of the 'SECOND" test, the cpmputer does not see the correct reported value, the P0401 code is set and your CEL comes on.
Once the code is cleared, the test is done two times before a failure is declared and a code is set so it will fool you if the drive cycle is not done twice.
Do the diagnostic work correctly, use known good Ford parts and the issue will be resolved.
Good luck.
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Freshman User
Joined: Nov 2008
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P0401 Pain in da Butt!!
Thanks, Bluegrass for info!! I have a few questions?
1. How do I ck to see if port behind EGR is clear? I removed EGR and ran a stiff piece of wire in there.
2. Why I feel EGR is good. I pulled vacuum on EGR and it held and with it off and while truck was running it made RPMs change.
3. How do you test DPFE?
4. How can you tell if the exhaust tube is open and clear? Do you remove EGR and start truck?
5. The EVR is new and hoses are tight? Has 12volts on one side of connector and 34 ohms between two.
Thanks again for better understanding of this system and info!!
1. How do I ck to see if port behind EGR is clear? I removed EGR and ran a stiff piece of wire in there.
2. Why I feel EGR is good. I pulled vacuum on EGR and it held and with it off and while truck was running it made RPMs change.
3. How do you test DPFE?
4. How can you tell if the exhaust tube is open and clear? Do you remove EGR and start truck?
5. The EVR is new and hoses are tight? Has 12volts on one side of connector and 34 ohms between two.
Thanks again for better understanding of this system and info!!
Lead Driver
Joined: Sep 2006
Posts: 7,907
Likes: 143
You can't test a DPFE with any relibility unless you have very good test equipment specific to the purpose.
The device senses the difference in pressure drop across an orfice in the metal tube to such a tight limit that you can't tell anything by static voltmeter testing.
The orfice in the metal tube is about .040 in diameter.
A check of exhaust relitive pulses from each DPFE tube on each side of the orfice should tell you if it/ they are open or plugged.
Opening the EGR in place on an idling engine should make the engine idle rough if the port into the manifold is open.
What happens is some exhaust gas is let back into the intake and upsets the A/F ratio enough to observe the result as a rough running idle.
If you can't find any problem outside the DPFE, I would get a new Ford DPFE and bet it solves your problem.
Reason for using a new Ford part is the very tight limits the part has to measure against being very close to 2% +/- and the reason so many have the 401 codes so often. Trying to rely on a used DPFE is a crap shoot at best. It may be all corroded inside from the exhaust acids and soot/gum.
This tight test limit is a mandate of the Federal EPA a number of years ago.
Good luck.
The device senses the difference in pressure drop across an orfice in the metal tube to such a tight limit that you can't tell anything by static voltmeter testing.
The orfice in the metal tube is about .040 in diameter.
A check of exhaust relitive pulses from each DPFE tube on each side of the orfice should tell you if it/ they are open or plugged.
Opening the EGR in place on an idling engine should make the engine idle rough if the port into the manifold is open.
What happens is some exhaust gas is let back into the intake and upsets the A/F ratio enough to observe the result as a rough running idle.
If you can't find any problem outside the DPFE, I would get a new Ford DPFE and bet it solves your problem.
Reason for using a new Ford part is the very tight limits the part has to measure against being very close to 2% +/- and the reason so many have the 401 codes so often. Trying to rely on a used DPFE is a crap shoot at best. It may be all corroded inside from the exhaust acids and soot/gum.
This tight test limit is a mandate of the Federal EPA a number of years ago.
Good luck.
Senior User
Joined: Aug 2009
Posts: 304
Likes: 2
I'm having the same problem on a 99 5.4l. Truck runs fine but I have the p0401 code. What is making me suspicious is it came on right after I cleaned my IAC valve. Coincidence? I've checked all vacuum lines. If I put
vacuum on the egr it almost kills the engine. I hate to throw parts at it
without knowing for sure. Any suggestions?
vacuum on the egr it almost kills the engine. I hate to throw parts at it
without knowing for sure. Any suggestions?
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