Another P0401 Thread

[Inept]

So, some fun testing tonight.

Since I had the EGR position sensor probe still in the connector, I ran a jumper into the car and placed my multimeter where I could see it. I also made use of the meter's min/max function for the first time!

I figured I could watch the position voltage and maybe see when the EGR test ran. Turns out - I can!

When I first started driving, the EGR valve was quite active. I saw it go from full closed (0.8V) to very briefly full open (4.4), but usually stayed in the range of 1-3 while puttering around.

Once I got it to the test conditions (above 50, top gear, decelerate until test runs) I could see the valve close for a bit, then it would jump up to ~2.7v for a second, then back to closed. Generally I could make it run twice after a fresh code clear and sometimes a clear and turn the engine off, and on the 2nd one it would turn on the SES. After the SES turned on, it seems like the computer gave up & just left the valve closed for the rest of the drive.

At one point, I went into a store for about 10 mins. After that, the valve started working again (pretty sure the SES was on at this point) until the test ran again, and then it stayed closed again.

It seemed to be pretty consistent at that 2.7V peak during the test, which is only about half open. Unless it peaks above that & my meter wasn't fast enough to catch it. Maybe that's not enough opening? or it isn't opening fast enough?

I may try to get more serious about it & see if I can record the manifold vacuum & pressure on Torque while running this. It'd be nice if I could monitor the EGR position on torque also, but I'm not sure if I can.

So... It's interesting that the valve is being commanded around and is working, though maybe not correctly enough? This may mean the PCM is ok, and the valve is the culprit like Cheyne suggests.

Inept

[billr]

What meter function were you using to read EGR voltage, AC or DC? And, if it was AC, we have to guess how accurate it is on a non-sine wave that is quite a bit faster than 60 Hz.

Does anybody know, for sure, what parameters the PCM uses to determine EGR flow is "low"? I'm thinking either the MAP or B1S1 O2 have to be intimately involved (maybe both), so I those two sensors are still possible suspects.

[Inept]

I'm pretty sure the EGR output is just DCV (of course, what the heck do I know? I've been wrong about a lot of stuff so far!). It probably does use PWM to command the valve on one of the pins (The ground transistor for Pin A discussed earlier?), but the position sensor has to wait for the pintle to actually move, so it shouldn't be a high frequency update.

I'm thinking 5V DC in, across a variable resistor for the position sensor = something DC out as the signal.

Everything I've read says it's just the MAP sensor that is responsible for determining if enough flow is entering the manifold. I haven't seen anything but speculation about the o2 sensor being involved.

Refer to the FSM page posted earlier:

here is the full text of the 2002 fsm page on dtc p0401.

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.

Looks like I was wrong about being able to run the test twice between ignition cycles, I coulda sworn I did it that way.

Inept

[Sid]

Interesting. My service manual (1996) says the PCM will close the EGR for the rest of the ignition cycle after a diagnostic test fail (DTC). It looks like that is what you saw when your SES light came on. I was wondering what the manual meant by an ignition cycle. Thanks for the info!

Also, the pintle position does use 5v DC (again, for the 1996). Mine is a little under 1 volt when the EGR is closed and increases as the EGR opens.

My guess is that pin A of the EGR is controlled by the PCM and the schematic from the service manual included in this thread is misleading. It's easier to sink (pull low) a voltage above 5 volts than it is to try to source a higher voltage from a microprocessor output. (unless there is some cheap driver that I don't know about. It's been awhile!)

Now, why do you get a P0401? That's the mystery.

Eric

[ultravioletnk]

Originally Posted by Inept

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?

you have to be somewhat careful to heat the parts up quickly without overheating the pcb and melting it, which is definitely more difficult on the tab of those transistor and switch packages. i'd suggest a hot air gun for the cleanest results. it would definitely be worthwhile to have a spare pcm on hand, but i think repairing the fault (if it is indeed that ic) is easy enough and very worthwhile, at the very least because it proves we can repair pcms rather than just replacing them.

Originally Posted by Inept

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.

do you have ac? if so you do want a pcm programmed for it. i know the pcm can be programmed with/without power steering, but i think the only difference is the control for the evo solenoid. i've deleted p/s on two of my cars and never got any codes due to that solenoid being disconnected. abs and cruise are "if the car has it, definitely make sure the pcm does" options. the really annoying one is the air injection system, if you get a pcm that's programmed for it and the engine doesn't have it, you will get persistent codes. there have been a few threads where people were trying to get rid of the codes after deleting that system, but i don't think anyone ever had success. i'm sure it is possible though.

i already mentioned the transmission details, you can run an auto pcm with a manual just fine but it will throw a bunch of codes about the transmission sensors and solenoids that aren't there. manual pcms were built without the transistors to run the solenoids entirely. another note: cruise usually works with an auto pcm regardless of the transmission, but if you've got an mp2/dohc or mp3/sohc transmission combo, the 2nd-3rd gen manual pcms won't like the different gear ratios and will refuse to engage cruise. on phoebe i've also gotten a periodic ses light due to the mismatched gear ratios, however i've yet to figure out what code it is because it's an obd1 car and it doesn't seem to store the code.

Originally Posted by Inept

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

see about the price for a spare pcm at a u-pull yard, and try swapping it out. you'll have to do the passlock relearn procedure as i believe someone already mentioned in this thread. if your code goes away, you can be pretty sure that the problem is that switch ic or the logic that controls it, and then you can try your hand at replacing it! or you could send me the faulty pcm and i'll give it a shot, i'm pretty decent at soldering and i'd like to try some pcm repair anyway.

if the code sticks around with a different pcm, you can be quite sure the problem is further down the line, although i have to say you've done quite thorough diagnosis and it does seem like all the "ducks are in a row" as it were.

[billr]

This is getting to be a long thread, so remind me... how thoroughly has the MAP sensor been checked? My perception is that is has only been checked to the extent that the KOEO reading correlates pretty well to your altitude.

How about the easiest check, remove the MAP sensor and make sure its inlet port and the port in the manifold are not choked with crud. Maybe the MAP sensor is damped and just to slow-acting for what the PCM is expecting when the EGR is opened.

[Inept]

Yes, I do have AC, and AIR & Power Steering, I think that's probably the only options - no cruise, no power windows, locks, no ABS, Manual Trans, SOHC.

BillR, I was kind of revisiting the MAP sensor in my mind also - I did clean it out when I cleaned the manifold. There was about 1/4" of crud in the whole manifold that I scraped & cleaned out. I did clean the sensor too - sprayed carb cleaner in the sensor hole, so I think it's as clean as it's going to get.

For testing, I did correlate the atmospheric pressure as you noted. I also hooked up a vacuum pump (mityvac) and measured the voltage on the signal pin as I changed the vacuum level. - mentioned in posts 4 & 5.

That's an interesting thought about the sensor being slow though- my testing certainly wasn't quick, and there's definitely a time component to the EGR test.

So I found a couple of yards nearby that have Gen3 Saturns - I'll see if I can sneak over to one and see if I can nab an EGR valve and MAP sensor for testing, and maybe a PCM if it looks promising with the right-ish options.

Inept

[Inept]

Ok, lemme 'splain.

No, there's too much - let me sum up:

I think it's fixed, it was the EGR valve - like Cheyne said.
And the PCM really didn't break, though I was really thinking it was the problem. I just had trouble wrapping my head around a brand new part not working, even though I've seen several references to bad aftermarket valves. - living up to my name I guess.


Now I'll 'splain.

I made it down to the junkyard where they had three 2002 Saturn S series!

I figured I'd give a map sensor and an EGR valve a shot - and walked out of there for only $20. Not too shabby. I asked about the PCM - it would be 24 if I needed it.

So, the long story is - I changed the Map sensor - no changes at all. My Rockauto EGR valve still gave the P0401 error, with the same symptoms as before (run through 2 tests after the codes were cleared, SES comes on with P0401 after the 2nd test).

I put the Junkyard valve on (it was the best looking of the 3 with the smoothest pintle action) and measured 0 volts at pin C. As we've learned, this should be around 0.8 or so with Key on, Engine off. After driving around for a bit, trying to force an EGR test - I had no luck. It always showed 0 volts, and eventually threw a P0405 code - low EGR signal.

Today, I went back to the junkyard and they were nice enough to let me swap for another valve. The of the 2 that were left, one was really carboned up, and one had a P/N for a 2nd gen valve. I took the carboned one, and on closer inspection saw that it had a screened gasket on it.

This is good - it means that 1) somebody put it on, either pre-emptively or as part of a fix 2) the valve is probably newer than 20 years old since it doesn't have the GM p/n stamped on it (Neither did my original one). And 3) if somebody put that screened gasket on, they were probably smart enough to make sure the problem was fixed - so the valve may be in good shape and functional.

Anyway, I cleaned up the valve and the screen, put it on the car and went for a drive.

Again, I saw on my voltmeter measuring Pin C that the EGR test ran twice after the codes were cleared. This time though, no SES, and the EGR kept working after the 2nd test! The EGR DTC was not set to complete though according to the Torque app.

So after doing a drive cycle, I pulled over & shut off the engine for a few mins, then started it up & headed for home - again, I saw the EGR test run, and this time , the EGR DTC immediately said complete, with no SES!

Now my only DTC not completed is the Evap, which I think I'll need to do a cold start & Idle for a while to complete - the engine wasn't quite cold when I started tonight.

I'll update with an addendum after all my DTCs complete & I get a successful emissions test.


I sincerely want to thank everyone who has been following along and offering help and advice - billr, Sid, ultravioletnk, Waiex191. I think we've all learned some stuff, and hopefully the wall of text here will help someone else in the future.

And if anyone needs a map sensor, let me know.

Inept.

[Sid]

Good to hear!!

I missed something. Are you monitoring pin C (pintle position) to determine when the EGR flow test is being run? If so, does the pin C voltage go higher on your working EGR than on the non-working one?

Thanks. I'm trying to figure out if the EGR is not opening far enough pass the flow test or is it blocked by carbon buildup.

Eric

[Inept]

Yes, I'm monitoring Pin C - and the voltage actually is a little lower on this functional valve - ~2.4V during the test that I can see.

The RockAuto valve behaved the same with both MAP sensors - so I don't think it's the MAP, though I haven't tried the Old MAP with JY valve. And I never set up the Torque App monitoring to watch the MAP pressure while the test ran. so I really don't know what the problem with the RockAuto valve is.

Maybe 2.4 volts on the JY valve represents a more open state than the other one, so there is more flow? The Rockauto one is new, so it's definitely not carbon inside the valve, and the rest of the passage is the same for both tests.

Or maybe the test is happening too fast for me to see the highest signal point. Anybody want to send me an oscilloscope?

I finally got all my DTCs marked as completed, but now I have a P0446 code, and I just changed the Evap vent valve by the gas tank (the old one was sticky and intermittent in my bench testing). I'll start a new thread on that one later maybe.

Inept

[Sid]

Wow. The plot thickens. So the solenoid/pintle is moving on both EGR's but one EGR passes the flow test and the other doesn't.

Here's another one: I put in one of my aftermarket EGR valves that I know has problems. In the meantime, I put in a new fuel filter. I had a gasoline smell coming from the engine compartment, couldn't find a gas leak, had the new fuel filter, so decided to put it in. That fixed the gas smell. I don't know why. So, the EGR worked for about 15 minutes and the SES light came on as I expected it would. After a while it went off and came on again. Then went off again and has been off since! Maybe Saturn karma? Or is there a connection?

I didn't take the time to check the DTC code but on my car when the SES light comes on it's virtually always an EGR DTC code.

[Inept]

That sounds like a fun one too.

Regarding the gas smell - I've read that these filters also have a pressure regulator in them, which is why they cost an arm and a leg. Maybe your regulator was passing too much pressure causing a fuel seep at a connection in the engine compartment.

I'd question this SES as an EGR - you mentioned, and my tests showed that once the SES is lit, it stops trying to run the EGR. So I'm not sure why the light would go on and off. Maybe the bulb is loose?

Ah, it never ends.

I didn't buy this car to restore it, but it's kind of interesting chasing the gremlins. At least it's not my primary driver so I can leave it alone if it stops being fun for a while.

[Sid]

The SES light went off and on over several ignition cycles. Sorry I wasn't more specific. I've had this happen before but they never stay off for this long.

I think I found an EGR gasket with a screen and ordered one. Here's a link for reference:

https://www.rockauto.com/en/moreinfo...&pt=4972&jsn=1

[Sid]

Well, the fuel filter "fix" didn't last. Now I have a P1406 DTC, pintle position error.

[Inept]

That looks like the one that was on my junkyard find - so I got a $10 screen with a free EGR valve

[billr]

Are there two of you, with similar EGR problems, using the same thread here? That is getting confusing.

Sid, you should start your own new thread.

[TomM96]

@ ultravioletnk
> to the left of the "MC33291DW" chip

In mid '90s i messed around with a Motorola PWM chip , pretty sure
it was MC3329...

Re PCM issues:
> Thank You, BillR for mentioning "Flagship1". They are located
within an hour's drive of me on L.I.
> on their webpage, they advise ->

Before sending us your unit for ECM repair, we recommend you test & replace the following:

Ignition Module & Coils
Ignition Control Module & Coils
DME Relay or Main Relay
Spark Plugs
Fuel Injectors
Idle Control Valves
TDC and RPM Reference Sensors
Air Flow or Mass Flow Meter
Throttle Position Sensor
Power Supply (Ford ECMs mostly)
Grounds <<<----------------------------------------------<<<
Check Engine Light DTC
.................................................. ..

I was going to suggest, maybe clean/refit the engine compartment grounds.
I had irregular idle and issues with the security system on my 2000SW.
After thorough/obsessive grounding initiative, those issues all resolved.

And about voltage readings: on my '97, snugging/cleaning the eyelet screw
securing the eyelet from B+ cable to UHJB increased voltages in UHJB
about a half of a volt.

Good call by Cheyne !