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Old 03-20-2022, 01:57 AM   #1
Inept
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Wrench Another P0401 Thread

Hi there!

Short time lurker, first time poster.

I acquired a 2002 SL last month with 237K on the odometer and have been beating issues into submission one by one. This site has been a gold mine of information, thank you to all who have contributed solutions & knowledge! It's been great reading and has helped me immensely!

My current issue is the dreaded P0401 code (insufficient EGR flow) that just won't go away. In the process of the aforementioned fixes, I've had the intake manifold off (manifold gasket) and cleaned about 5 pounds of crud out of it, including the EGR passage (which was pretty much closed off).

Last night, I pulled the exhaust manifold & found the AIR passages almost solid with carbon, so I cleaned all of those, along with the small AIR holes into the head.

I'd previously replaced the EGR valve with an SKP from RockAuto (which may be the current issue).

I've run the car with the valve removed, it revs up & bounces off the rev limiter at 4K pretty well, there's good suction on the vacuum EGR hole, and lots of exhaust coming out of the exhaust EGR hole (and some carbon flakes at first after the recent exhaust manifold/AIR passage cleaning.

I tested the EGR valve voltages according to this guide:
https://easyautodiagnostics.com/gm/4...-valve-tests-1
And everything tested ok, as far as I could test with a multimeter.

After all of this, I still got a P0401!

Tonight, I realized that I hadn't thoroughly cleaned the passage through the head, so I pulled the exhaust manifold again, and was able to use a shop vac to suck a string through the passage and use it to pull knotted bits of shop rag through both the intake and exhaust passages, so there should be plenty of flow, even if it may not be pristine in there.

The car runs great and idles fine at around 900 or so (indicating no major vacuum leaks).

So to summarize:
EGR passages are not blocked, new EGR valve that seems to test good, No vacuum leaks, Still seeing a P0401- that's the only code that's getting thrown (using Torque Pro).

I am resetting the codes after each round of work just to make sure the code is actually coming back, not just still hanging around.

What else can I look at? Maybe a funky MAP sensor (reads 13.6 with the key off)? Does the O2 sensor have anything to do with EGR (running in closed loop)? Could my brand new EGR valve be bad & not opening correctly? Bad PCM making the EGR valve not work right?

I feel like I'm running out of EGR system parts to check!

I believe I'll need to get this SES light off to pass emissions soon, so I'm a little motivated.

Thanks for any thoughts on the subject!
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Old 03-20-2022, 10:35 AM   #2
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Default Re: Another P0401 Thread

I replaced my EGR valve last year. No P0401 code, but it was running like the EGR valve was not behaving. When I pulled the electrical connector to the EGR, it was happy again. Not relevant to your journey, except I bought a Standard Motor Products EGV544 EGR Valve and I'm happy with it:
https://www.amazon.com/gp/product/B000C7ZFKQ

There have been strong arguments for the MAP sensor being the device that detects insufficient flow. The other possibility is the O2 sensor.

Welcome to the forum first time poster!

http://www.saturnfans.com/forums/sho...d.php?t=212681
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Old 03-20-2022, 01:14 PM   #3
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Default Re: Another P0401 Thread

So, what units is that 13.6 MAP reading??? If that is psia and you are at a high altitude, then it may be reasonable.

However, if you are at/near sea-level, or the reading is inches Hg (either absolute or vacuum), then the sensor sure seems bad and should be checked/cleaned/replaced.
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Old 03-20-2022, 04:27 PM   #4
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Default Re: Another P0401 Thread

Thanks team,

I just checked the MAP reading - ignition on, engine off - and it's saying 13.2PSI right now - that converts to 26.9 inHG

I'm at 2700 feet, and some random website says the current (station, absolute) air pressure in my area is 27.3 inHG, or 13.4PSI, so it's not too far off.

Thanks for that thread Waiex191, I'd seen it, but had forgotten it. Sounds like pretty much where I'm at - unfortunately there's no resolution on it!

I also found this one, conveniently named the same as mine, right after I posted with quite a lot of info.
http://www.saturnfans.com/forums/sho...d.php?t=238073

I'm going to measure some MAP sensor voltages just for fun & see where that takes me.
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Old 03-20-2022, 09:12 PM   #5
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Default Re: Another P0401 Thread

Ok, just got done checking some things -

1) My warm engine idle speed is 750RPM, not 900 as I previously stated - I just knew it was under 1K & guessed...

2) I'm fairly confident in the health of my MAP sensor - applied vacuum with a hand pump & measured voltage on the signal wire & got a nice straight line of data that was slightly lower than & parallel to the data from here: https://www.aa1car.com/library/map_sensors.htm

I'd show a chart, but can't figure out how to post a pic real fast - you'll just have to take my word for it!


So I feel like I'm back to the EGR valve itself. Can I put 12 volts on Pins A & E and see if the pintle retracts all the way, or would that fry something?
http://www.saturnfans.com/forums/att...5&d=1526244236

Thanks!
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Old 03-20-2022, 11:59 PM   #6
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Default Re: Another P0401 Thread

Don't forget this from reply #2...

"The other possibility is the O2 sensor"
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Old 04-06-2022, 11:52 PM   #7
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Default Re: Another P0401 Thread

Ok, I haven't disappeared and I still have the code.

I have been doing further research, and found an interesting diagnostic video that's not on a Saturn, but on a car using the same EGR valve. YoutubeLink

One thing he said is that if you apply power to the EGR valve solenoid (pins A & E) while the car's running, the pintle will open fully & the engine should die right quick due to the extra air coming in & leaning it out. And you can do that with the solenoid unplugged, so you don't have a danger of frying the PCM.

So I did that, and with the engine off, you can hear the pintle clicking open & closed (if you do it with the solenoid removed, you can watch it). With the engine running, powering that circuit kills the engine very rapidly. Like within a second or 2.

This tells me that the EGR valve can open enough, and my passages are sufficiently clean.

So I started rechecking things, and the voltage coming through the EGR connector is ~9.24 volts (Key on) which I thought was pretty close to 10-12, but it turns out that 9 volts just ain't enough to run my valve.

I found some wiring diagrams that show that the solenoid power circuit should be getting full battery voltage - in my case 12.2-12.4 volts, a long way from 9.2.

Further accidental experimentation shows that I have about 9 volts on EGR connector pin E even with the key off! This ain't right - it should be 0 since it's a key switched circuit.

I suspect that 9 volts is leaking somewhere, now I just need to go tracing that circuit.

The saga continues.

Oh, in regard to the O2 sensor, I can't find anything that mentions it in relation to the EGR, plus Torque pro shows nice O2 sensor readings - I think all of my hardware is ok (map, EGR valve, O2, passages, etc). It sure looks like electrical at this point.

Off to warm up the multimeter again!

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Old 04-07-2022, 01:15 AM   #8
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Default Re: Another P0401 Thread

Ok, after some more looking & probing, I found a way to get a wire into the back of the EGR connector Pin E (12V). So now I can monitor voltage there with it plugged in or ...

My brilliant thought was that I could put 12V on it, bypassing all the other connections and plugs in the circuit. Then when the PCM commanded the EGR open by grounding Pin A, there would be plenty of power to open the valve.

I did that, and I heard something move inside the EGR valve. I think it's the pintle opening somewhat, since the car won't start with 12V applied to Pin E. Remove the jumper wire, and it starts as normal.

So much for that idea.

I still think Pin A should be getting battery voltage, but now I'm thinking maybe there's a short to ground somewhere, so it's opening up when it shouldn't.

BUT, there's still that mystery 9V on the plug at pin E when disconnected, however it disappears (gets sent to ground?) when I plug in the connector (That probe wire into connector pin E came in handy). Maybe I should try to measure the current from pin E through the solenoid coil...

Stumped, gonna sleep on it.

Also, my emissions test notice came in the mail this week. Yay.

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Old 04-12-2022, 07:14 PM   #9
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Default Re: Another P0401 Thread

My EGR pin E has about 14 volts on it when the car is running (if this helps). Also, the service manual (1996) says the PCM will open the EGR sometime during deceleration in high gear and check for a change in the MAP to verify the flow through the PCM. Just an FYI. It sounds like you've verified the MAP is working anyway. Also, the pin E (B+) comes from the UHJB (underhood junction box) and there is a 10 Amp fuse there. Have you checked that?
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Old 04-12-2022, 08:40 PM   #10
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Default Re: Another P0401 Thread

Thanks for that, Sid. I was expecting that to be the case - battery/alt voltage on Pin E.

I found a schematic HERE that shows key on battery voltage going through the EMISS fuse directly to the 2nd O2 sensor, the Evap Purge valve and the EGR valve.

I have confirmed that with the key on, I get batt voltage at the specified pins of the O2 sensor connector and the Evap Purge connector, but not the EGR connector. If I pull the 10amp EMISS fuse, those voltages go to zero (but the EGR connector still shows the mysterious 9.25 volts).

I also checked continuity with the fuse pulled - O2, Evap & fuse connection are all connected, nothing on the EGR connector Pin E.

So that seems to point to a problem between the UHJB and the EGR connector - which I should be able to bypass with just a jumper.

However, when I do that (12V to Pin E directly) with the valve plugged in, I hear the EGR click open, and the car won't start due to the giant air leak I just created.

The EGR should be controlled by the PCM grounding Pin A, which tells me that Pin A should NOT have continuity to ground normally. (that part of the schematic isn't available for free! I DO have continuity PinA to ground all the time, so may be a problem too.

So maybe I have a broken wire in the fuse to connector wire and/or a problem in the PCM for the EGR control(ground).

I'm really not sure what to try next!


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Old 04-12-2022, 11:32 PM   #11
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Default Re: Another P0401 Thread

Disconnect the PCM from the wiring and try applying 12V to the EGR. If it still seems to actuate, then you probably have a short in the harness; if no actuation (with PCM disconnected), then the PCM output is likely shorted. Have you peeked at live-data to be sure the PCM is not commanding that EGR "on" at all times?
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Old 04-13-2022, 10:18 AM   #12
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Default Re: Another P0401 Thread

Thanks BillR, that's a good suggestion.

For diagnostics, I have Torque Pro on my phone with a dongle in the OBD2 port - I'm not sure I can see if the EGR commands with that setup.

I haven't dug into the PCM/harness area yet since there's a lot of wires there & it's pretty intimidating!

I'll give it a shot when I get some free time in the next day or 2 to see what happens.

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Old 04-13-2022, 11:37 PM   #13
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Default Re: Another P0401 Thread

Ok
PCM removed:
No connection between ground and Pin A of the EGR connector (there was connection when the PCM was in).
No connection between Emiss Fuse & Pin E (Never have seen continuity here, but I think there should be).
Virtually 0V (10's of mV) on Pin E when key is on or off (was 9.25V when PCM connected).

This still confuses me, since the schematic I have shows pretty much a direct connection between the battery and pin E with the key on, so should be 12V key on and 0V with key off.

Applying 12V to Pin E does NOT activate the EGR valve when the PCM is removed.

I'm leaning more toward bad PCM, except for the lack of 12V at Pin E with the key on.

Now to try and track the wire from UHJB Emiss fuse to the EGR connector.
Anybody know how to lift up the UHJB so I can look under it? or have a good way to trace that wire?

Alternately, I can try to get a Junkyard PCM and swap it in, then if still no joy, just run a dedicated 12V wire to the EGR.
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Old 04-14-2022, 12:20 AM   #14
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Default Re: Another P0401 Thread

New news, I found continuity from each EGR connection to a location in the large PCM connector, so it seems that Pin E does NOT connect to the Emiss Fuse.

I think this definitely points to something wrong in the PCM itself.

It also shows that the schematic I was looking at earlier is not necessarily correct - I'm glad I didn't spend the 10 bucks to get access to the whole thing.

I also just found DifferentRacing's page on the P0401, and hidden in the middle of the big paragraph, it says this: "The PCM controls the solenoid by supplying it ignition voltage whenever exhaust gas needs to be recirculated into the intake manifold." This supports what I found with the connectors.

P0401 Page

Since I've found good continuity with all the pins in the EGR connector, I really don't know what else could be the problem aside from the PCM.

Am I missing anything before I order a PCM?

Thanks for hanging in there with me!
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Old 04-14-2022, 10:37 AM   #15
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Default Re: Another P0401 Thread

I know the EGR valves are different on the gen1/gen2 than your gen3; but does anybody know if the basic control/wiring is the same? I am tempted to go do some checking on one of my gen1s. Meanwhile, I will do some searching for schematics.
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Old 04-14-2022, 11:11 AM   #16
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Default Re: Another P0401 Thread

I'm not sure - I also found this on the DifferentRacing page:

EGR Harness Pinouts

It looks like the inputs to the EGR valve are the same as long as you have a 5 pin connector, but who knows if the other end of the wire ends up in the same place.

Can I post a picture here? I could show where I found continuity on the PCM connector if needed.

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Old 04-14-2022, 12:47 PM   #17
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Default Re: Another P0401 Thread

With the PCM connected, and the EGR valve disconnected, check the key-on voltage at pin E of the (EGR) harness connector. Does it now show ~12V when not loaded?

You mentioned testing for current in that circuit, I think that might give us some clues.

Note that the EGR valve is not "on-off", but is PWM. If the PCM, or some other fault, is pulling pin A hard to ground, then the feed to pin E may be marginal to provide solid 12V at E. I mean "marginal" as designed, since it might not have been intended to handle the current resulting from constant ground at A.
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Old 04-14-2022, 03:10 PM   #18
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Default Re: Another P0401 Thread

After reinstalling the PCM, I get the same results I had before:
Pin A has continuity to Ground
Pin E has 9.18V with Key off, 9.24v with Key on.

These are measured with the connector disconnected from the EGR valve

As to currents,

with a little creative wiring, I can measure the current through the Pin A wire with key on & off:
Key on:8 uA (yes, micro Amps)
Key off:8 uA.

I knew there wouldn't be much, since I haven't had any battery issues. for this test, pins B,C,D were all disconnected, which may or may not affect anything. i.e. the connector was out of the EGR valve, and I used jumpers for pins A & E to get it through my multimeter.

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Old 04-14-2022, 05:01 PM   #19
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Default Re: Another P0401 Thread

Did you connect the ammeter to harness pins A and E? That is not the way to test current in this circuit. You need to put the ammeter in series with the A pin and socket (or E pin and socket); you need to have the solenoid coil resistance there to limit current. In this case, before EGR is commanded on, A was not grounded so current was limited by the internal resistance of the meter.

My understanding is that you only get about 9V at the E pin of the harness, even when the valve is disconnected; no load on that 12V supply. You need to do the same check at the appropriate pin of the PCM; even opening up the PCM to check soldering of that pin to the PCM board and probing for voltage right at the solder pad. If you cannot get the normal 12V at the PCM itself, with no load, then it seems certain the PCM will have to be replaced before fussing with anything else.

Pin-outs of the PCM connectors are not readily at-hand for me, but I can find them if necessary.
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Old 04-14-2022, 06:26 PM   #20
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Default Re: Another P0401 Thread

Quote:
Originally Posted by Inept View Post
New news, I found continuity from each EGR connection to a location in the large PCM connector, so it seems that Pin E does NOT connect to the Emiss Fuse.
it definitely does connect to a fuse, on 1st/2nd gen cars it's called "EGR", not "EMISS". i do seem to recall the 3rd gen cars with air injection have an "EMISS" fuse, i'm not sure if that also runs the egr or not. here is the section of the schematic that shows the egr. this is for the 2nd gen cars, 1998 in particular, but it's very similar if not the same on 3rd gens.

Quote:
Originally Posted by Inept View Post
I think this definitely points to something wrong in the PCM itself.
not likely, while it is possible for the pcm to fail, generally replacing it results in no change in whatever the problem was, at somewhat significant expense. if the fuse and egr valve are known good, i would really suspect corrosion somewhere... that would particularly explain the weird low voltage you're seeing on the b+ pin.

Quote:
Originally Posted by Inept View Post
It also shows that the schematic I was looking at earlier is not necessarily correct - I'm glad I didn't spend the 10 bucks to get access to the whole thing.
yeah, those sites tend to be full of bull excrement. the only schematic i really trust is the official one. i seem to recall i have an fsm that includes the '99-02 (3rd gen) cars in a virtual machine, i'll spin it up and get you the 2002 schematic if so.

Quote:
Originally Posted by Inept View Post
Since I've found good continuity with all the pins in the EGR connector, I really don't know what else could be the problem aside from the PCM.

Am I missing anything before I order a PCM?
check very thoroughly for corrosion, especially on the underside of the uhjb. disconnect the harness plugs from the underside of said junction box and test for continuity at those pins, both to the egr and the pcm. i really don't think the pcm is your problem.
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