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Old 01-26-2016, 10:50 AM   #1
blentz
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Default can't rid myself of P0401

I have a 2001 SC2 with 187k miles on it. Recently I've been having issues with a recurring P0401 code for insufficient EGR flow. So far, I've tried:

- Removing the EGR valve and running the car, the idle went up to 3.5KRPM and was very loud, seemingly both passages are clear
- I've run compressed air through the EGR to intake port with the throttle body removed, flow seemed good to me, it doesn't appear this passage is blocked at all
- I've tried snaking a wire through both ports without much luck, I read somewhere online about a GM service procedure with a speedometer cable attached to a drill to help remove any build up
- The EGR valve has been replaced; it's a Delphi EG10176 that I got from Amazon for about $70. I believe it's the same as the OEM one I removed
- I have removed the MAP sensor and made sure it and it's port was not full of crud

Each time I clear the code, it comes back after a half dozen drive cycles. I have no driveability or noticeable gas mileage issues. No hesitation or stumbling. No other DTCs are set, only P0401.

I live in New Hampshire and we have yearly OBD smog testing. If I cannot get this resolved I fear it will impact the roadworthiness of the car. It's ridiculous to think I would have to take the car off the road because I can't get MAP sensor to read the correct change in intake pressure.

Should I try putting a new MAP sensor in it? The consensus on here seems to be that the MAP sensors seem to never go bad in these cars. Should I try a third EGR valve assuming the replacement I got is defective (but isn't setting any of the codes for the EGR position sensor?)

I can't think of any other diagnostic procedures to perform and I'm at the point where I'm considering to continue to throw parts at the car and hope something sticks.

I appreciate any and all suggestions you folks can provide.
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Old 01-26-2016, 10:52 PM   #2
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Default Re: can't rid myself of P0401

I compared my map sensor against another 2001 SC2. The car that's throwing P0401 codes has one that looks like a 213-796, but the car that's running fine has 21024034 stamped on it. I've had the car for many years and I've never had to change the MAP sensor before, but, according to the info online, it looks like the car has the wrong MAP sensor on it.

In the attached picture, the left one is on the car throwing P0401s and the one on the right looks like the one on the 'control' vehicle.


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Old 01-27-2016, 03:26 AM   #3
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Default Re: can't rid myself of P0401

P0401 is for insufficient egr flow. This isn't a map sensor error.

Under normal circumstances when removing the egr valve and starting the engine, the exposed intake port (one side, the other is the exhaust side) acts as a large vacuum leak and lets the engine rev to maximum - 4500 rpm. This is the programmed limiter with the engine parked in neutral as the engine computer knows the car isn't driven so it limits revs to 4500 and cycles the injectors on and off to prevent the engine from revving any higher. At speed and depending on whether a manual or automatic, speed limits are either 104 or 124 mph (6500 rpm). While you saw only 3500 rpm, this may be a partially blocked intake port not allowing more air in to raise engine rpm to programmed rev limits.

The map sensor is probably fine and not causing the egr error code.
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Old 01-27-2016, 10:44 AM   #4
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Default Re: can't rid myself of P0401

The P401 is set when low flow is detected by a less than calculated intake vacuum decrease while decelerating in top gear with throttle closed. The RPM limiter is at ~4k RPM and as you are not reaching this the intake side is restricted. Clean out intake side with carb cleaner soak with engine off. Allow it to air out and soak for 1/2-1 hour before starting with EGR off and maybe it will clear up.
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Old 01-31-2016, 05:37 PM   #5
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Default Re: can't rid myself of P0401

I have a SL1 and for several months. I have been cleaning and I even replaced the EGR valve. However,I still get the *******' p0401 code. My car needs to pass emissions but I cannot get this code to go away. I started with 4 codes and this is the only one I cannot seem to get rid of. Please help!
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Old 02-01-2016, 01:15 PM   #6
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Default Re: can't rid myself of P0401

Quote:
Originally Posted by OldNuc View Post
The P401 is set when low flow is detected by a less than calculated intake vacuum decrease while decelerating in top gear with throttle closed. The RPM limiter is at ~4k RPM and as you are not reaching this the intake side is restricted. Clean out intake side with carb cleaner soak with engine off. Allow it to air out and soak for 1/2-1 hour before starting with EGR off and maybe it will clear up.
Thanks for the tips, OldNuc and fdryer! I recently coded the same and am now wondering which of the 2 holes is the intake side, if I'm looking in at the EGR from the driver side fender. I assume it's the hole the pintle is over, but figured I'd ask because I'm well versed in EGR function.
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Old 10-13-2019, 05:44 PM   #7
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Default Re: can't rid myself of P0401

Ugh, years later, still intermittently struggling with this... I was unaware anyone had replied!

I had falsely assumed the blockage was on the exhaust side, not the intake side. Since the vacuum leak caused by removing the valve was making it rev - BUT not making it rev all the way to the point of hitting the rev limiter I am sort of inferring the *intake* side is partially blocked, not the *exhaust* side.

I might be being thick here, but I think fdryer and OldNuc are saying that removing the EGR valve and starting the car should result in the engine revving all the way to the rev limiter -> it should repeatly hit the 4000 RPM limit over and over if the intake EGR passage is totally clear, and if it doesn't quite hit the 4000 RPM rev limiter, the passageway between the intake manifold and ERG mating surface is partially blocked.

I recall the intake was pretty caked with oily goodness the last time I had the throttlebody off for cleaning, but it never occurred to me that it could be clogged to this extent.

Thank you for giving me one more thing to try.

97RedSL1 and any google users - the exhaust side is passenger side, intake on driver side - Here's a photo of a plate I made to cover the intake side while I was trying to diagnose and focused on the exhaust side - in the picture, the exhaust port is open.

Attached Images
File Type: jpg 20190807_171407.jpg (213.2 KB, 73 views)
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Old 10-15-2019, 03:04 PM   #8
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Default Re: can't rid myself of P0401

I have a 97 sc2 with 91k miles.im getting P0404 I put a new egr in. Ran it with the valve off and the hit 4k and the rev limiter kicked in. I'm also getting p1624. Wtf is that
I cant get this damn light off!
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Old 10-16-2019, 09:50 AM   #9
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Default Re: can't rid myself of P0401

Quote:
Originally Posted by blentz View Post
Ugh, years later, still intermittently struggling with this... I was unaware anyone had replied!

I had falsely assumed the blockage was on the exhaust side, not the intake side. Since the vacuum leak caused by removing the valve was making it rev - BUT not making it rev all the way to the point of hitting the rev limiter I am sort of inferring the *intake* side is partially blocked, not the *exhaust* side.

I might be being thick here, but I think fdryer and OldNuc are saying that removing the EGR valve and starting the car should result in the engine revving all the way to the rev limiter -> it should repeatly hit the 4000 RPM limit over and over if the intake EGR passage is totally clear, and if it doesn't quite hit the 4000 RPM rev limiter, the passageway between the intake manifold and ERG mating surface is partially blocked.

I recall the intake was pretty caked with oily goodness the last time I had the throttlebody off for cleaning, but it never occurred to me that it could be clogged to this extent.....
Yes, with egr valve removed, the intake port is a direct feed to the intake manifold and should allow the engine to rev up immediately upon startup to programmed rev limit (4k).
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Old 10-16-2019, 10:15 AM   #10
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Default Re: can't rid myself of P0401

Quote:
Originally Posted by timl865 View Post
I have a 97 sc2 with 91k miles.im getting P0404 I put a new egr in. Ran it with the valve off and the hit 4k and the rev limiter kicked in. I'm also getting p1624. Wtf is that
I cant get this damn light off!
With two error codes and replacing the egr valve to correct P0404, the two errors may remain until several engine/drive cycles ensure one or both aren't active errors before the self diagnosing OBD II system resets one or both codes. P1624 seems to be treated to inadvertently pressing the cruise control button three times with a 30 second freeze frame snapshot of all sensors. This electronic snapshot is stored in memory for a reader to display sensor signals. Simply resetting this error code will also reset P0404. If the egr valve replacement corrects P0404 then either letting the OBD II system reset itself after a few drive cycles or you can manually reset both error codes with either a reader reset button or disconnecting battery negative for 10 seconds or more. Error codes are stored in live memory so removing battery power erases them but if any error caused by a faulty sensor isn't corrected, the error returns.

P1624; http://www.saturnfans.com/forums/sho...d.php?t=223409
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Old 04-03-2020, 04:47 PM   #11
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Default Re: can't rid myself of P0401

I was just chiming in, as I was following this thread as yet ANOTHER person who has a P0401 Code. I've had it now for 2 years. I've just changed the EGR Valve and cleaned up the throttle body (just for good measure). The performance definitely impoved as well as the idle smoothness. I also now notice a slight gray smoke when starting it. Could have been some carbon knocked loose along with the massive amounts of throttle body cleaner I used to clean out the muck. The EGR was 100% in need of changing as it seems to have been 1/2 inch thick of guck, and there was no way the throttle body cleaner could get through there. Considering I found a Delphi EGR for $50 including new metal gasket, I couldn't go wrong.
It doesn't affect the driving abilities or affect the accellaration, but after clearing the codes, it returned after about a month. Doesn't make any sense.

So, i'm just curious if anyone else has had any luck on this, or if it's going to be filed under "Unsolved Mysteries". My personal mechanic even said people have junked cars over this (not Saturns, but other makes). I'm not discouraged, but perplexed eternally.

Hope to hear someone have a good solution.
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Old 04-03-2020, 05:38 PM   #12
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Default Re: can't rid myself of P0401

That code, for "Insufficient EGR Flow" is too vague to be of much use. There is no meter/sensor in the system to actually measure flow, so the PCM has to infer "insufficient flow" from what is happening in the exhaust, as reported by the O2 sensor(s). Without knowing the algorithm the PCM uses to determine flow, nearly anything on the engine could be a suspect. Diagnosing the real cause takes more patience and equipment than is tolerable for most people; shops included.

Considering just strictly the EGR system, you could have bad wiring or a bad PCM.
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Old 04-03-2020, 07:34 PM   #13
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Default Re: can't rid myself of P0401

Just for kicks, try disconnecting the battery, and reconnecting. I watched a ScannerDanner video, where the PCM didn't re-learn it's 0/closed voltage value until power was cut. Key-off didn't do it.
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Old 04-03-2020, 08:14 PM   #14
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Default Re: can't rid myself of P0401

P0401 isn't really vague and the map sensor is what measures it. When the pcm sees that the voltage indicates that the EGR pintle has moved as commanded but there is not sufficient decrease in vacuum as seen by the pcm via the map, the P0401 insufficient flow detected will be commanded. This test is performed during a normal deceleration.

This will also not be related to Paul's video on the P0404 pintle position error.

Best way to begin with this is to remove the EGR valve, start the engine briefly to see if the engine revs all the way to the 4K limiter. If it does, you will look for a restriction on the exhaust side. If it does not, you will look for one on the intake side.
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Old 04-03-2020, 08:31 PM   #15
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Default Re: can't rid myself of P0401

Quote:
Originally Posted by Cheyne View Post
P0401 isn't really vague...

...not be related to Paul's video on the P0404 pintle position error.
Oops, I didn't notice the difference in code numbers.
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Old 04-03-2020, 09:13 PM   #16
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Default Re: can't rid myself of P0401

Cheyne, do you have access to the PCM source-code, do you know that all it does is look at the change in MAP, and know the exact values?

I'm thinking that the PCM also looks at the O2 sensor and takes into consideration spark timing, rpm, TP, IAT, and CLT to determine how much the MAP/O2 should have been affected by the EGR flow introduced. How much difference will there be if the engine has compression on the low end? What is the difference in the changes if you are running 87 octane E10 with frequent knock retard as opposed to 92 octane "E0" fuel? Then there is exhaust back-pressure (crudded-up cat) that can be in the equation.

And it is all just a bunch of guessing unless we do know the PCM programming.
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Old 04-03-2020, 09:41 PM   #17
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Default Re: can't rid myself of P0401

Bill, I don't know that I've ever seen anyone here that has the actual source code. So, without getting into the weeds, here is what we do know.

During a normal deceleration event, the pcm must see:
Engine rpm between 1350-2000 rpm
Vehicle speed greater than 30 mph
Throttle or TP is closed
ECT is greater than 150F
Vehicle is in top gear
There are no EGR pintle, TP sensor, MAP, VSS, IAT, ECT or IAC errors

Based on the above, the pcm sees that all parameters for testing are set and all sensors operating correctly. The pcm then commands the EGR to open and monitors it via the map sensor. If the vacuum is not sufficiently reduced, the pcm sets the P0401.

The parameters of back pressure, octane, compression will be within parameters of the original map signal before the pintle is commanded open and verified by EGR pintle position voltage by the pcm. It is then looking for a change within threshold of that original map signal.

We have a first time posting OP. This error code is almost always due to a blockage in the EGR passages or perhaps sometimes a severely clogged EGR that managed to not throw a pintle position error.

Your questions and parameters are obviously valid, in particular for a vehicle without a map sensor. This does not, however, appear to be the way the Saturn logic with the map is written according to the engineers that wrote the FSM.
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Old 04-03-2020, 10:24 PM   #18
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Default Re: can't rid myself of P0401

I agree the EGR valve or passages are the most common causes, but the (recent) OP was asking how the problem could persist when those had been "fixed".

All the sensors you mentioned below, except for VSS, can fail "in range" so that the PCM is ignorant of the failure and will mistakenly calculate an EGR flow error.

"There are no EGR pintle, TP sensor, MAP, VSS, IAT, ECT or IAC errors"

It is not an "unsolved mystery", just incomplete investigation.
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Old 04-03-2020, 10:36 PM   #19
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Default Re: can't rid myself of P0401

planetrj did not say that those problems were fixed. He or she stated that they had replaced the EGR valve and cleaned the throttle body. This leaves the EGR passages.

I agree this is just an incomplete investigation. But with the EGR replaced, it needs to begin with the passages. Removing the EGR and observing the limiter indicates on which side to begin.
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Old 04-04-2020, 12:27 AM   #20
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Default Re: can't rid myself of P0401

OK, I missed that. Agreed, if the passages in the head/manifold have not been cleaned out, then that should be the very next step.
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