P0401 EGR Flow Insufficient Mystery Solved
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Newbie
Joined: Jul 2009
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From: SF Bay Area
P0401 EGR Flow Insufficient Mystery Solved
Hi guys, I know this has been discussed several times, but I think I have something new to contribute to the topic.
TL/DR: it was the MAP sensor o-ring.
So my car started throwing the p0401 error, and I started troubleshooting the issue with the help of this forum (mainly the search function). The troubleshooting was super annoying, because the SES light usually came up in ~30 miles after the battery disconnect, and you need to disconnect the battery to get to the EGR valve. Ok, so over the span of 2 months I have:
SES p0401 was still coming up, so that last point was the moment of despair. I have replaced 3 components in a 3-component system (valve, lines, solenoid) - and the system still was not working properly.
But then I asked myself a question: how does the EGR system test itself? Where is that sensor that actually tells ECU that EGR flow if insufficient? What does insufficient even mean here?
And finally, in one of the infinite p0401 threads, I have 'discovered' that EGR system actually has 4 (not 3) components. The 4th component is the MAP sensor (the one on top of the throttle body). I grabbed the 8mm socket wrench, removed the sensor - and there you go, the o-ring has actually turned into a c-ring! The damn MAP sensor was leaking pressure - not enough to throw off other codes, but enough to be unable to detect the difference in working EGR system under boost. I have replaced the o-ring with a similar piece of rubber (not a perfect fit, but enough to test the hypothesis) - and voila, the SES light is not coming up, and my OBDII scanner says EGR system is finally 'OK'. Overall, I have spent $240 on EGR valve, $80+$240work on EGR solenoid, $20 on new silicone lines, learned to remove the EGR valve in under 10 minutes with my eyes shut, cleaned up the inside of the engine - and the actual problem was with $1 o-ring that takes 30 seconds to replace. Well, maybe this write-up will help some other poor soul with a similar issue. Enjoy.
TL/DR: it was the MAP sensor o-ring.
So my car started throwing the p0401 error, and I started troubleshooting the issue with the help of this forum (mainly the search function). The troubleshooting was super annoying, because the SES light usually came up in ~30 miles after the battery disconnect, and you need to disconnect the battery to get to the EGR valve. Ok, so over the span of 2 months I have:
- Cleaned up the EGR valve
- Ordered and installed a replacement valve ($240)
- Ordered 10ft of replacement vacuum lines, and replaced all the EGR vacuum lines (PITA to reach from the top of the engine, but I managed)
- Asked SpeedElement guys for an consult (they re-routed the vacuum lines and tied them with zip ties, but could not find anything being wrong with the setup)
- Ordered a replacement EGR solenoid ($80) and installed it ($120) at the SpeedElement (it is so much of a pain to replace that I let the people who own a car lift to do the job)
- Removed EGR valve once again, and thoroughly cleaned the EGR inlets in the engine block
- Replaced the new EGR valve with the old one, since I was now 100% sure that the original EGR valve was perfectly functional after I cleaned it.
SES p0401 was still coming up, so that last point was the moment of despair. I have replaced 3 components in a 3-component system (valve, lines, solenoid) - and the system still was not working properly.
But then I asked myself a question: how does the EGR system test itself? Where is that sensor that actually tells ECU that EGR flow if insufficient? What does insufficient even mean here?
And finally, in one of the infinite p0401 threads, I have 'discovered' that EGR system actually has 4 (not 3) components. The 4th component is the MAP sensor (the one on top of the throttle body). I grabbed the 8mm socket wrench, removed the sensor - and there you go, the o-ring has actually turned into a c-ring! The damn MAP sensor was leaking pressure - not enough to throw off other codes, but enough to be unable to detect the difference in working EGR system under boost. I have replaced the o-ring with a similar piece of rubber (not a perfect fit, but enough to test the hypothesis) - and voila, the SES light is not coming up, and my OBDII scanner says EGR system is finally 'OK'. Overall, I have spent $240 on EGR valve, $80+$240work on EGR solenoid, $20 on new silicone lines, learned to remove the EGR valve in under 10 minutes with my eyes shut, cleaned up the inside of the engine - and the actual problem was with $1 o-ring that takes 30 seconds to replace. Well, maybe this write-up will help some other poor soul with a similar issue. Enjoy.
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Joined: Aug 2005
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From: West Coast
Thanks for sharing, that sounds like a pretty deep troubleshooting rabbit hole you had to go down. Its been shown that our egr valves can start to leak at ~80k miles so your replacement was probably not in vain.
EDIT: noticed you put the old one back on, just make sure its not leaking at all when its suppose to be closed, because that will melt the ISCV.
EDIT: noticed you put the old one back on, just make sure its not leaking at all when its suppose to be closed, because that will melt the ISCV.
Evolving Member
Joined: Jan 2015
Posts: 250
Likes: 15
From: Hub City
Excellent, a little more regarding Mitsubishi's Egr.
Long story short, it is looking for a pressure difference when it activates the solenoid.
If it doesn't sense a pressure difference via MAP, it will thrown a code, and the car will be behave erratically
Long story short, it is looking for a pressure difference when it activates the solenoid.
If it doesn't sense a pressure difference via MAP, it will thrown a code, and the car will be behave erratically
Thread Starter
Newbie
Joined: Jul 2009
Posts: 3
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From: SF Bay Area
Biggiesacks, thank you pointing out the EGR leak problem, I did not know that. I'll replace it again at the first opportunity - I need to re-zip-tie the vacuum lines anyway.
lonestarr20, excellent summary - this is exactly what was happening.
lonestarr20, excellent summary - this is exactly what was happening.
Interesting, thanks for the heads up on this. I went through similar diagnostic steps and couldn't figure out my EGR issue so I ended up disabling it through the ECU.
Do you know the part number for the MAP sensor o-ring or size?
Do you know the part number for the MAP sensor o-ring or size?
Thread Starter
Newbie
Joined: Jul 2009
Posts: 3
Likes: 5
From: SF Bay Area
I could not find the PN for the o-ring, so I just ordered a set various sizes of of o-rings for $13 on Amazon (figured that I will be able to find one that fits).
Good thing they offer a lot of sizes - the o-rings have two dimensions, inner diameter and thickness (or tube diameter). Two of the o-rings that would fit the sensor would not allow to push it back into the engine, so I had to go with small/thinner o-ring after all. I used this set: https://smile.amazon.com/gp/product/B0765BD3KF/.
Good thing they offer a lot of sizes - the o-rings have two dimensions, inner diameter and thickness (or tube diameter). Two of the o-rings that would fit the sensor would not allow to push it back into the engine, so I had to go with small/thinner o-ring after all. I used this set: https://smile.amazon.com/gp/product/B0765BD3KF/.