Another P0401 Thread

[ultravioletnk]

Originally Posted by billr

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.

it isn't, 1991-1994 (except cali emissions in 1994, they went to the linear egr with a somewhat rare and desirable adapter for 1994 cali cars) use a vacuum egr that's merely actuated by a separate solenoid. the egr wiring is fundamentally the same for all 1995-2002 cars, and 1994 cali emissions cars.

[Inept]

Thanks BillR, yes the ammeter was in series with the circuit:

Connector Pin E -> {Meter} -> EGR Valve Pin E -> {solenoid} ->EGR Valve Pin A - > Jumper wire -> Connector Pin A

Meter read 8 uA key on or off. When I opened the circuit, the meter went to 0 uA.

By the way, solenoid coil resistance is 8-9 ohms, as it should be (new EGR valve).

As I mentioned, when the PCM is in the car, Pin A is grounded at all times (and I don't think it should be).


Yes, you are correct, I get 9V at the connector Pin E all the time - key on,or off - It should be (I think) 0V Key off, 12V Key on. Sid mentioned above that he has 14V on Pin E when the car is running - i.e. Alternator charging voltage.

Regarding the PCM pinout, I know which pins connect to the EGR, since I traced them from the EGR connector to the large PCM connector yesterday. I have no idea how I would probe for voltage on that thing though. I did notice that there are 6 screws on the PCM itself & thought about opening it up, but thought it might void the warranty.

Actually, I didn't know if it was easily openable without destroying it, or what I'd find inside.

Maybe if I have a good PCM on hand, I'd give a shot to opening the old one for posterity.

Inept.

[ultravioletnk]

Originally Posted by ultravioletnk

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.

well, apparently on the 3rd gens, the egr is indeed powered directly from the pcm... which seems like a boneheaded decision to me. it's on a fuse for good reason on the cars with electrical systems designed by people who actually had brains! this discovery makes me think you may well be on the right track in suspecting the pcm, and also drastically lowers my opinion of the 3rd gen pcm design (which i already never really liked).

here is the official 2002 schematic which confirms the egr is fed directly from the pcm. i plan to get all the 3rd gen fsms put up on my site to complement the 1998 one i host, at some point in the future. for now i just threw that page of the schematic up.

Originally Posted by Inept

Actually, I didn't know if it was easily openable without destroying it, or what I'd find inside.

Maybe if I have a good PCM on hand, I'd give a shot to opening the old one for posterity.

Inept.

i have one that i've opened, it's not a very serviceable construction but it would be possible to repair. the case is sealed with rtv so once you get the screws out you will have to cut through the rtv to get it open. the board is a flex pcb with a conformal coating, so soldering to it will be more than a bit annoying. it would be neat to identify the fault and repair it, as these modules aren't exactly being mass-produced anymore.

[billr]

I agree, there is not much you could do to repair it, other than heating and reflowing a cracked solder joint. However, confirming that 12v for pin E is gone right at the PCM itself would confirm the PCM is bad.

I had forgotten the gen3 PCMs are "underhood" and had to have sealing of the case and conformal-coating of the board. Yeah, that would make it a bit more of a pain,

If you do decide to replace the PCM, I recommend this:

https://www.fs1inc.com/2002-saturn-s...plug-play.html

You will get a unit that is as "known-good" as possible for your car. No core charge, so you can the keep the old one to play with. I found that vendor to have a rather worrisome ordering/shipping process, but the unit arrived on-time and worked perfect. I think others here have ordered from them (Flagship 1) successfully, too.

[Inept]

Thanks team for sticking with this!

It's nice to have confirmation about the EGR Pin E running through the PCM, I knew most of the EGR pins did, so I just started probing in the PCM connector since it was off.

Any ideas for probing for 12V at the PCM? I think the huge connector would have to be plugged in, and I'd have to figure out how to find the right wire out of about 50 (it's red right? ). I have 0 ohms between the PCM connector and the EGR connector on that wire, so as long as it's making good connection to the PCM, I should be good. Maybe I'll squeeze that pin connection a bit just for kicks.

Honestly, the condition of this car is pretty decent - I just had to replace some wear items - very little corrosion or damage to be found, especially for 237K on the clock. Those connectors are pretty well sealed and all the pins look brand new. As we've discovered, everything seems to end at the PCM.

Maybe I'll try cracking the thing open. I'll have to think on that. If I do, I'll have to figure out how to put up some pictures!

Inept

[billr]

"Back-probing" into one half of a mated connector is usually fairly easy, using a sewing needle or even paper-clip as an "extension" to the meter probe. You slip the needle in until it stops and it will almost always be touching some part of the wire/crimp at the contact inside. Worst case, you use the needle to pierce the wire insulation just slightly back from the connector. That small puncture will pretty much close up when you remove the needle so that moisture resistance is maintained, but you could tape over it too.

Unfortunately, probing the external connector may not give a final verdict on the PCM; that final connection may be the problem. Assuming you still get low voltage at the (PCM) wire for (EGR) pin E, I urge you to open up the PCM and test at the solder pad for that PCM pin. Be sure to use a meter probe sharp enough to pierce through any coating on that pad/pin.

[Sid]

Originally Posted by Inept

Thanks BillR, yes the ammeter was in series with the circuit:


As I mentioned, when the PCM is in the car, Pin A is grounded at all times (and I don't think it should be).


Yes, you are correct, I get 9V at the connector Pin E all the time - key on,or off - It should be (I think) 0V Key off, 12V Key on. Sid mentioned above that he has 14V on Pin E when the car is running - i.e. Alternator charging voltage.

Inept.

Inept, my car is a gen II (1996). There is 14 volts on Pin E of the EGR all the time and the PCM pulls pin A lower to open the EGR solenoid/pintle. It looks like it's different than the gen III. I've watched the DC voltage on my EGR Pin A while driving. It goes lower during acceleration (opening the EGR valve/pintle). It never goes below 6 volts or so. That would be 8 volts across the EGR solenoid coil. Which would be about 1 amp of current. It looks like the gen III PCM drives pin E high to open it but I don't know why you would see 9 volts there with pin A grounded and drawing almost no current through the solenoid.

Incidentally the pin A on my EGR is a pulse-width modulated signal running at about 130 Hz. I was watching it with a DC volt meter.

[Inept]

Thanks again Sid,

Yes, the big questions for me are why Pin A is grounded all the time thru the PCM, and why I have 9V (when disconnected) on Pin E all the time, key on or not.

Since all this goes through the PCM, I'm feeling fairly certain that it's the culprit.

I found one a local junkyard - same year, engine & Trans, for $85.

I'm thinking hard about opening mine up, and if I kill it, there's a replacement 20 miles away...

Inept.

[ultravioletnk]

Originally Posted by Inept

Thanks again Sid,

Yes, the big questions for me are why Pin A is grounded all the time thru the PCM, and why I have 9V (when disconnected) on Pin E all the time, key on or not.

Since all this goes through the PCM, I'm feeling fairly certain that it's the culprit.

my vague guess would be failure of whatever driver transistor modulates the signal to pin A, which may have also resulted in some kind of failure in the feedback circuit the pcm uses to determine the voltage on pin E. it is, however, possible that 9v on pin E is normal when the EGR is disconnected with the 3rd gen pcm, i'll check on janus. if that's the case the failure may just be the driver transistor for pin A, which would actually be pretty easy to replace. as i mentioned, i already took one of the 3rd gen pcms apart, i'll dig it out and do some investigation.

[ultravioletnk]

Originally Posted by ultravioletnk

it is, however, possible that 9v on pin E is normal when the EGR is disconnected with the 3rd gen pcm, i'll check on janus. if that's the case the failure may just be the driver transistor for pin A, which would actually be pretty easy to replace. as i mentioned, i already took one of the 3rd gen pcms apart, i'll dig it out and do some investigation.

i unplugged the egr on janus and checked the voltage at pin E, 9.33v. so it appears that's a totally normal voltage to see at the egr connector while unplugged on 3rd gens. janus has no problems whatsoever with the egr, no codes and it passes emissions great. i would note, the fsm states that the pcm controls the solenoid by supplying ignition voltage, which makes me think that pin A should in fact be grounded at all times. the schematic confirms this.

edit: here is the full text of the 2002 fsm page on dtc p0401. are you really, completely, 100% sure the egr passage isn't blocked?

The exhaust gas recirculation (EGR) valve consists of a pintle valve which moves by the use of an internal solenoid and an internal position sensor (potentiometer) used to determine pintle valve position. The powertrain control module (PCM) controls the solenoid by supplying it ignition voltage whenever exhaust gas needs to be recirculated into the intake manifold. The PCM monitors the EGR solenoid and position sensor circuit for correct voltage levels at certain times. The PCM also monitors the flow of exhaust gas through the EGR valve by using the manifold absolute pressure (MAP) sensor. During deceleration, the EGR valve is normally closed. However, the PCM will run a flow test by opening the EGR valve and monitoring the change in the MAP signal voltage. DTC P0401 sets when the change in the MAP signal voltage is below a certain threshold when the EGR valve is being commanded ON during the deceleration.

DTC Parameters

DTC P0401 will set if the PCM determines EGR flow to be restricted based on engine speed and barometric pressure using the MAP sensor when:

The engine speed is between 1250-2600 RPM.
The vehicle speed is greater than 56 km/h (35 mph).
The throttle position (TP) angle is 0 percent (vehicle in decel).
The engine coolant temperature (ECT) is greater than 70°C (150°F).
No cam, crankshaft position (CKP), ECT, EGR solenoid, 5-volt reference, idle air control (IAC), intake air temperature (IAT), MAP, misfire, PCM internal fault, TP or vehicle speed DTCs have been set.
DTC P0401 diagnostic runs once per ignition cycle for 1 second during deceleration once the above conditions have been met.

DTC P0401 is a type A DTC.

Diagnostic Aids

Verify the MAP sensor reads correctly at idle (between 1-1.5 volts). If the MAP sensor is skewed or faulty, DTC P0401 may set.

The most likely cause of this DTC is carbon buildup around the EGR valve or a blocked EGR passage.

Remove the EGR valve and check for a blocked passage and for carbon buildup around the EGR pintle valve. Clean or replace the EGR valve and/or clean EGR passage.

[ultravioletnk]

Originally Posted by Inept

Yes, the big questions for me are why Pin A is grounded all the time thru the PCM, and why I have 9V (when disconnected) on Pin E all the time, key on or not.

to summarise my previous post, that is because those are 100% normal conditions to see on a 3rd gen pcm. after doing my own testing on janus, with a known good egr system, and reading through the 2002 fsm, i really don't suspect the pcm. your most likely problem is a blocked egr passage, i know you did clean those out but perhaps there's still more crud in there.

[Inept]

Ultravioletnk, thank you so much for all of that!

Apparently I can put that 9v out of my mind - I was very convinced that it was a problem.

Did you happen to see if you had continuity between EGR connector Pin A and ground while disconnected? Mine has continuity at all times - key on or off. That might also point to a failed transistor.

Regarding the passages, I'm *pretty* sure they're clean:
1) I've done a lot of scraping on both the intake & exhaust side, got lots of carbon out.
2) I've sprayed a couple of cans worth of B12 carb cleaner through them
3) I can put a hose on each of the EGR ports can and blow through them with very little resistance
4) I can stick a shop vac hose on the EGR port & suck a piece of string through all the way, & have drug pieces of shop towel through both passages.
5) I've started the car with the EGR off, I get lots of air (exhaust) coming out the exhaust port, and the car revs to 4300 right away. the first couple of times I did get a few chunks of carbon blown out the exhaust port, but it seems to have stopped. In related news, the AIR passages in the exhaust manifold were almost completely blocked - they're clean now.
6) If I start the car with the EGR installed but unplugged, I can apply 12V through the solenoid to open the pintle & the car dies immediately (I've seen this as a test for blocked passages on several youtube videos.

Thinking back to the MAP sensor, I'd eliminated that back in posts 4&5 based on what I see on my TorquePro App. I know I did some voltage probing too, but I evidently didn't record it.

As I see it, the only parts of this system are:
EGR Valve
Map Sensor
PCM
Wiring
Connectors
EGR passages

And we've looked at all of them at some point - the PCM is just the most unclear at this point.

I've been trying to figure out if there are any other diagnostics I can do with what I have. I'm pretty sure my TorquePro is read only. Or if a FLAP store has a bidrectional scanner I borrow/rent send commands to the EGR... I'll have to ask at Oreilly's or autozone.

Thank you so much again for the input - it's really invaluable to have a working system to test!

Inept

[billr]

I'm keeping pretty much quiet now, since it seems the gen3 does have different electrical control than my gen1, but I gotta say... you have already observed good exhaust flow and manifold vacuum at the two EGR ports, when uncovered (yes, very subjective as to what is "good"), and applying 12V to pin E does give sufficient flow to quickly kill the engine. I am not convinced this is only crud remaining in the EGR passages.

[ultravioletnk]

Originally Posted by Inept

Did you happen to see if you had continuity between EGR connector Pin A and ground while disconnected? Mine has continuity at all times - key on or off. That might also point to a failed transistor.

if you look at the schematic, pin A is directly tied to ground in the pcm. just in case you weren't aware, (⏚) is the symbol for a connection to ground.

Originally Posted by Inept

Regarding the passages, I'm *pretty* sure they're clean:
1) I've done a lot of scraping on both the intake & exhaust side, got lots of carbon out.
2) I've sprayed a couple of cans worth of B12 carb cleaner through them
3) I can put a hose on each of the EGR ports can and blow through them with very little resistance
4) I can stick a shop vac hose on the EGR port & suck a piece of string through all the way, & have drug pieces of shop towel through both passages.
5) I've started the car with the EGR off, I get lots of air (exhaust) coming out the exhaust port, and the car revs to 4300 right away. the first couple of times I did get a few chunks of carbon blown out the exhaust port, but it seems to have stopped. In related news, the AIR passages in the exhaust manifold were almost completely blocked - they're clean now.
6) If I start the car with the EGR installed but unplugged, I can apply 12V through the solenoid to open the pintle & the car dies immediately (I've seen this as a test for blocked passages on several youtube videos.

well i'd say that qualifies as "really, completely, 100% sure".

Originally Posted by Inept

As I see it, the only parts of this system are:
EGR Valve
Map Sensor
PCM
Wiring
Connectors
EGR passages

And we've looked at all of them at some point - the PCM is just the most unclear at this point.

it does seem the pcm is most suspect, and it certainly is possible there's some fault within it. i'll take some pictures of the one i have taken apart soon, and see if i can't trace out which component the supply pin for the egr connects to. regarding a replacement pcm, do you have any "u-pull" yards nearby? $85 seems like a lot, i seem to recall that a pcm is $40 at the u-pull-and-pay around here.

Originally Posted by billr

I'm keeping pretty much quiet now, since it seems the gen3 does have different electrical control than my gen1, but I gotta say... you have already observed good exhaust flow and manifold vacuum at the two EGR ports, when uncovered (yes, very subjective as to what is "good"), and applying 12V to pin E does give sufficient flow to quickly kill the engine. I am not convinced this is only crud remaining in the EGR passages.

your input is still very worthwhile, and as reiterated the passages do seem quite clear... i actually didn't realise the 3rd gens controlled the egr so differently myself, until i went digging in the fsm and checked on my own 3rd gen car. pcm failures are quite rare in my experience but it is entirely possible!

[Cheyne]

The aftermarket egr valve is likely the problem. Try cleaning and installing the original now that you’ve cleaned the passages.

[ultravioletnk]

Originally Posted by ultravioletnk

as i mentioned, i already took one of the 3rd gen pcms apart, i'll dig it out and do some investigation.

all right, here are some pics. sorry about the gigantic sizes, the forum doesn't seem to have a good way to display thumbnails while still allowing for full resolution. the following 3 are an overview of the pcm internals:





and here is the switch ic that runs the egr valve. it's the one on the left, i haven't probed out what the one on the right controls. it's one of the two 5-legged packages with the middle leg clipped, to the left of the "MC33291DW" chip. i scraped the coating off to be able to read the part number better.



the part number for that ic is "BTS308" and it appears they're relatively inexpensive on ebay etc. here is the datasheet. if that component has failed, it should be relatively straightforward to replace, if you have some soldering skills.

Originally Posted by Cheyne

The aftermarket egr valve is likely the problem. Try cleaning and installing the original now that you’ve cleaned the passages.

i do always question/suspect aftermarket sensors/solenoids and the like, having been burned repeatedly by them. the solenoid is known to work fine, given that supplying it 12v directly causes the car to stall. perhaps it would be worthwhile probing the feedback pins on the egr to see if the potentiometer in the aftermarket one is working properly.

again, sorry for the gigantic photos. it seems to just be how this forum handles embedding full-resolution images.

[Inept]

Wow, busy weekend on the thread! I didn't get anything done with the car - it was a busy Easter weekend for me.

BillR, I appreciate your input and assistance - please continue to offer thoughts and ideas; it appears that we have different systems to play with but I think the more eyes on the issue we can get, the better! Thanks for hanging around! I don't know what else I can do on those passages, I think the restriction in the EGR valve is as small or smaller than anything remaining in there. I'm open for other cleaning ideas, but the engineers didn't make nice straight holes that are easy to get through!

Cheyne, I have read several posts about aftermarket EGR valves not being up to snuff but have never heard why/ do they not open enough? I ran the tests described in the page linked above (this one: Easy Auto Diagnostics ) and it passed all the tests.

Tests 4 & 5 describe testing the position sensor - it's been a couple of weeks since I did it, but I don't remember any problems. I'll recheck it this evening.

My old valve is not an option at this point, early on in the process, I figured I'd try to dismantle the old one & see if I could find a problem since I had the new one on hand, and they don't appear to be dismantle-able.

I also just checked a local pick a part - different than the one where I found the PCM - it looks like they do have a 2001 S Series, Maybe I'll go check that out if we really think the new EGR valve is a problem. Won't be helpful for the PCM though - I keep reading about the PCM being Year, Engine, Trans specific. Of course, if it's super cheap, maybe it's worth a shot?

ultravioletnk, Those are interesting pictures. I was afraid it'd be a surface mount transistor which is at the upper edge of my soldering capabilities. I could handle this though.

Just have to get up the nerve to bust it open...

Inept

[ultravioletnk]

Originally Posted by Inept

Cheyne, I have read several posts about aftermarket EGR valves not being up to snuff but have never heard why/ do they not open enough? I ran the tests described in the page linked above (this one: Easy Auto Diagnostics ) and it passed all the tests.

i think the main issue with aftermarket parts tends to be poor quality control, sometimes they're fine and sometimes they're absolute garbage. there is no doubt, the oem parts were built the best. sadly, there aren't many of those floating around anymore unless you really have deep pockets.

Originally Posted by Inept

Tests 4 & 5 describe testing the position sensor - it's been a couple of weeks since I did it, but I don't remember any problems. I'll recheck it this evening.

it definitely wouldn't hurt to go through it again and ensure that potentiometer is really reading accurately. i have found aftermarket tps units to be incredibly spotty, so potentiometers seem to be a significant weak point of the chinese manufacturers so much stuff is outsourced to these days.

Originally Posted by Inept

My old valve is not an option at this point, early on in the process, I figured I'd try to dismantle the old one & see if I could find a problem since I had the new one on hand, and they don't appear to be dismantle-able.

I also just checked a local pick a part - different than the one where I found the PCM - it looks like they do have a 2001 S Series, Maybe I'll go check that out if we really think the new EGR valve is a problem.

ah yes, ye olde destructive teardown... done a few of those myself. the best approach to u-pull yards is to just show up, pay the $2-3 entry fee, and go hunting. their inventory records are, uh, a bit lacking, and you really do best just looking around for yourself. also, i hear any item that fits in a pocket is free >.> they charge that entry fee for a reason. remember, they are a scrap metal business with a side hustle in used auto parts. they'll crush a whole car full of good parts in a heartbeat if they think the metals in it are more lucrative.

Originally Posted by Inept

Won't be helpful for the PCM though - I keep reading about the PCM being Year, Engine, Trans specific. Of course, if it's super cheap, maybe it's worth a shot?

pcms are not year specific, only gen/wiring harness specific. '96-early '99 are all cross-compatible, as are late '99-02. the transmission (auto pcms will run fine with a manual, at the expense of some persistent codes, manual pcms will never run an auto) and options (a/c, p/s, cruise, abs, air injection) are what really matter.

Originally Posted by Inept

ultravioletnk, Those are interesting pictures. I was afraid it'd be a surface mount transistor which is at the upper edge of my soldering capabilities. I could handle this though.

yes, it is a surface mount switch ic (not just a transistor, if you look at the datasheet it's purpose-designed as a switch) and being on a flex pcb you do want to heat the pads quickly, get the legs separated, and let them cool down again as fast as possible. it is nonetheless very achievable, although some practice on scrap pcbs would be a good idea. a hot air gun would probably help quite a lot to preheat the area. the tab (heatsink, tied to the clipped centre pin) will be the big challenge, you want to heat it up really fast to get it separated but not apply any more heat than is required. the 4 legs on the bottom can easily be handled by clipping the legs, then desoldering the remnants and cleaning up the pads. don't get ahead of yourself though, there's still some possibility the problem is not in the pcm.

[Inept]

So I tested the pintle position sensor again last night, Valve plugged in, key on, measuring voltage between Pin C & Ground
At rest (closed) I get ~0.8V
As I push the pintle in, it increases until I see about 4.8V. There is a bit of distance at the end, maybe 1/16 inch where the voltage seems to have maxed out at 4.8 and doesn't vary for that last bit.

?seems ok?

I didn't realize that the PCM was a flexible circuit board, I see it now that you mention it. I've never soldered on one of those before, but how hard can it be? Maybe $85 hard?

Might be worth asking what the pick a part place would charge for an ECM - the 85 was from a local "we pull it for you" yard.

You're the first one I've heard say the year isn't critical. Re: options, mine's an SL so it's not like I"m gonna miss out on anything if the PCM doesn't have a particular option.

So does anybody any ideas on what's next? I feel like I'm running out of things to look at!

Thanks!
Inept

[Inept]

Originally Posted by Inept

As I push the pintle in, it increases until I see about 4.8V. There is a bit of distance at the end, maybe 1/16 inch where the voltage seems to have maxed out at 4.8 and doesn't vary for that last bit.

Edit: 4.48V, not 4.8V.