DIY: 1MZ, EGR, code P0401 & P0402 diagnostic & repair

[Nervous]

Just want to share my experience about diagnosing and fixing popular MILs: P0401 and P0402. P0401 - insufficient EGR flow, P0402 - excessive EGR flow. I'll do it on example of 1MZ engine, 5S and 3VZ are pretty similar in this area.

These codes are caused by the same components, namely:
1) EGR valve,
2) EGR pipe #1,
3) EGR pipe #2 (V6),
4) EGR Vacuum modulator,
5) EGR VSV and vacuum hoses from throttle body to modulator, from modulator to VSV and from VSV to EGR valve.
6) 1MZ also has an exhaust gas temperature sensor mounted in the EGR valve, but I don't have information that it might be a cause of P0401/402. But still - it would be good to test it if all other tests did not discover the root of the problem.



There is a very good series of article about EGR and P0401 diagnosing techniques on example of 5SFE engine:
http://www.motor.com/magazine/pdfs/082009_04.pdf
http://www.motor.com/magazine/pdfs/092009_04.pdf
http://www.motor.com/magazine/pdfs/102009_04.pdf
http://www.motor.com/magazine/pdfs/112009_04.pdf
http://www.motor.com/magazine/pdfs/122009_04.pdf

I recommend to read all of them before starting throwing money in new parts.

So, briefly about my experience with P0402. It was about 4 years ago. I guess, this code is easier to diagnose than P0401 since it says "excessive flow". This automatically excludes clogging issue. It means that EGR valve is not closed when it should be. So, it could be sticking EGR valve or bad VSV. In my case it was bad VSV, which is very popular cause of both codes.

Now I had P0401 on my other Camry, on the Wagon I just restored. During the restoration I removed the EGR valve and cleaned it (I thought!), I cleaned the vacuum modulator (I thought!), EGR gas temperature sensor was cleaned as well. I also removed the EGR pipe #2 (the one that connects rear exhaust manifold to the engine block) and cleaned it, so it wasn't clogged for sure. I also checked the EGR pipe #1 (the one that connects engine block with the EGR valve) for clogs and it was ok. I replaced gaskets between these components (just in case). Nevertheless, I got the P0401 code.

Ok, starting to diagnose it. First of all, make sure all EGR-related rubber hoses are not cracked and not leaking vacuum. Replace any suspicious hose. Be careful buying rubber hoses from AdvanceAutoParts or AutoZone: they might have a little bit bigger inner diameter and they will be loose (ask me how I know).

After hoses are ruled out the next suspicion is the EGR VSV. It is probably the most common reason of P0401 and P0402 codes. Here is how it looks like on 1MZ:


Remove the plastic cover from the engine and remove the VSV (you'd need to loosen bolts which hold the rail with several VSVs (including the EGR one) and unbolt the blue VSV I marked as "other VSV" on the image:



3 steps to check the VSV:
1) Test resistance, should be around 36 Ohm
2) Apply 12V - should click
3) Using the vacuum pump apply 5Hg vacuum to one port of VSV closing another port by a finger. If this is a perfect VSV then it should hold vacuum.

When I was fixing the P0402 I found the problem very fast: the first VSV test failed, resistance was infinite.
With the P0401 it was more challenging, however. First two tests passed, but the third one didn't - it leaked vacuum pretty badly. "Here we are!" I thought. I was already about to order a new VSV (about $87, btw! (OEM)) but then I decided to make sure the VSV is a culprit. I have a second Camry with the same engine. I tested the VSV from my another Camry (the VSV there is relatively new, I replaced it when I was fixing P0402 several years ago) and it held vacuum perfectly. So, I just swapped VSVs in my Camrys and drove one and then another. After ECU reset the P0401 came back after about 30 miles on a highway, so, one commute. And, surprisingly, I got my P0401 back! Moreover, my other camry with the leaking VSV never gave me the code! Hmm, the lesson: even a leaky VSV may still do the job!

Next step. After ruling out the VSV the next suspect is EGR valve itself. Let test that it opens and closes, if vacuum is applied/released. I ordered a vacuum pump, MityVac at HarborFreight ($35), but I heard you can rent it at AutoZone too. So, disconnect the hose from the EGR valve and connect the vacuum pump to the EGR valve port. Start the engine. Slowly apply 5Hg vacuum. At this point the proper functioning valve should be opened and engine should start working roughly or even stall. If it didn't stall - release the vacuum and engine should work fine.
If this test failed it may mean the following:
1) EGR valve itself is bad, either clogged or membrane is bad;
2) EGR pipe is clogged.
You'll need to unbolt the valve:


and start the engine for short time. See if exhaust gas is coming from the EGR pipe (on the pic - below the air intake, rusted outlet). If it does - the pipe is fine. If not - need to unbolt the lower part of the pipe (PITA!) and clean it with carb cleaner and some kind of wired brush.

Now the EGR valve itself. Use the carb cleaner and spray into both ports:


(the connector is the exhaust gas temperature sensor I mentioned at the beginning).
Let it soak for several hours.
I actually did this the first time when I was restoring the engine on my wagon. And that time I completely forgot to check a small metal pipe that connects the EGR valve and vacuum modulator. This one:



This time I found out it is completely CLOGGED! Thus, my modulator had no chance to work at all. So, check it and clean it with carb cleaner. I just replaced it since I had a spare EGR valve already picked up off ebay some time ago.

To test the EGR valve off the vehicle:
1) connect the vacuum pump to the valve's port
2) spray carb cleaner into the bigger (upper) hole of the valve. It should NOT come out from the bottom (smaller) hole at this point
3) apply 5Hg vacuum - the carb cleaner should go out of the smaller hole.

Ok, I again thought I found the problem. Assembled everything back. Broke 1 stud and almost broke 2 others even though I used a torque wrench (9 ft-lbf):


Fortunately, has spare ones:




So, I'd recommend to replace the studs preventive and use new bolts too. Use anti-seize compound on every bolt and stud: this may save you a lot of nerves later.

Ok, test drive. After 30 miles the P0401 is back!
What's left? Modulator. Again, I used a benefit of having two identical cars - swapped modulators between two cars and drove another camry (the one that was perfectly fine before the swap). And, bingo, I got P0401 on her! So, the modulator is bad. Even though I cleaned the air filter in it and sprayed a lot of carb cleaner.


I tried to blow into the port P and it didn't pass any air! It was also clogged. Another modulator allowed air to pass from the port P. Thus, I found the final problem - the modulator. After I replaced it the code was gone and a day after I saw "EGR system complete" in ScanMaster OBDII software.

The last part that could be tested is the gas temp sensor. This sensor actually detects the insuff/excessive gas flow and the culprit might be there.
To test the sensor, remove it from the valve. Using an ohmmeter, measure the resistance between the terminals.
Resistance:
64 – 97 kOhms at 50C (112 F)
11 – 16 kOhms at 100C (212 F)
2 – 4 kOhms at 150C (302 F)
If the resistance is not as specified, replace the sensor. You can perform the test in boiling water (obviously you'll be able to check only first 2 resistances, for the 3rd one you need boiling oil; but first two might be enough).


So, if anybody wants to share his experience in nailing down P0401/P0402 codes, please, do not hesitate!

[swagger_wagon468]

damn man. lots of good info. now i tried to read most of the thread...lots and lots of reading. lol. i didnt see anything about the question im about to ask.

my CEL will come on, stay on for a day or 2, then go back off. sometimes it stays off for a day...sometimes a month. it just varries. any suggestion as to why this happens? maybr this could narrow down a bit better? (P0402 by the way)

[fenixus]

very good info! great job Nervous!

one thing to add.
my old 5s-fe modulator (very similar to 1mz) had a clogged backpressure port on bottom, basically i could see a black dot on inner side of filter in place where the backpressure port was (supposedly) making contact with the filter. the root cause was of course dirty (not completely clogged) EGR valve with its backpressure pipe dirty with carbon deposits.

also modulators have some sort of diaphragm on bottom which may get clogged too. moral, never bother cleaning filters or modulator's bodies, just get a new one.

also a note on getting replacement hoses (if not willing to get expensive OEM ones). I found very cheap metric diameter silicone hoses on Hong Kong ebay. Ones used on Toyota EGR systems are 3mm ID and 2mm wall thickness, same can be used also for the throttle opener, like this:
http://cgi.ebay.com/ebaymotors/1-8-3...item43a1aa8407

currently minimal order length is 3m (~10ft), but guy used to sell 1m pieces too in past.

it's a very cheap replacement and works fine. as you said those replacement vacuum lines from NAPA, Autozone and Advance Auto are simply too loose (SAE vs metric inner diameter makes difference on this one as SAE equivalent is more loose).

also, there is one more important step when troubleshooting the VSV for EGR, namely make sure the wire harness ground point on intake manifold is clean and secure!
even new perfect VSV for EGR will not function properly if it's not having a proper ground. I learned that the hard way on 5s-fe.

[CWH5]

This is a good DIY. I will also be making a very extensive but pretty simple DIY on these codes as well.

[peshwa]

Nice work on learning software programming :-) Good job as always Nervous. You are a devout Yota man....ha ha

[SilverSoarer]

The mechanical EGR system is one of the few weak parts of the 1MZ-FE design.

[73sport]

Originally Posted by Nervous

Just want to share my experience about diagnosing and fixing popular MILs: P0401 and P0402. P0401 - insufficient EGR flow, P0402 - excessive EGR flow. I'll do it on example of 1MZ engine, 5S and 3VZ are pretty similar in this area.

These codes are caused by the same components, namely:
1) EGR valve,
2) EGR pipe #1,
3) EGR pipe #2 (V6),
4) EGR Vacuum modulator,
5) EGR VSV and vacuum hoses from throttle body to modulator, from modulator to VSV and from VSV to EGR valve.
6) 1MZ also has an exhaust gas temperature sensor mounted in the EGR valve, but I don't have information that it might be a cause of P0401/402. But still - it would be good to test it if all other tests did not discover the root of the problem.

#6, its the temp probe that determines all of the above faults.
- Too cool when EGR is at full volume = insufficient flow (low heat).
- Too hot = excessive flow.
My MZ - was happy with cleaning all of the offending plugged passages (none were plugged solid but were reduced in size due to carbon) thus reducing flow and temperature.

As for cleaning, carbon is dried combustion gases. Picking out 95% of it with semi-flexible wire is far faster and much more efficient than chemical treatments (IMO). Use the chemical of choice for the last little bit just don't get it on the diaphragm or you can damage it. Then use a good gasket scrapper to clean the mating surfaces and steal gaskets.

The MZ intake plenum is hard to soak, so I highly recommend digging the carbon deposits out with a bent wire vs solvent. Sure the whole intake needs cleaned but it takes a large volume of solvent to immerse the affected areas.

A chunk if stainless steel wire, a shop vac and an air compressor make quick work of cleaning the tubes, valve, and intake plenum. Once you can see the internal areas are 95% free of carbon, NOW do the solvent thing. . .

Note: If the temp probe is left unplugged or has a broken wire you can produce an P-0401.

Just my 2 cents. . .

[Nervous]

Originally Posted by 73sport

#6, its the temp probe that determines all of the above faults.
- Too cool when EGR is at full volume = insufficient flow (low heat).
- Too hot = excessive flow.
My MZ - was happy with cleaning all of the offending plugged passages (none were plugged solid but were reduced in size due to carbon) thus reducing flow and temperature.

How the 5S-Fe determines P0401 / P0402 in this case? It doesn't have this sensor.

Thanks for additional info, btw!

[fenixus]

Originally Posted by Nervous

How the 5S-Fe determines P0401 / P0402 in this case? It doesn't have this sensor.

Thanks for additional info, btw!

MAP detects irreguralities in intake manifold absolute pressure (e.g. caused by a faulty EGR flow). it's far less precise than 1mz detection method. hence the EGR error detection problem on 5s-fe until it's really a big problem

on the other hand trouble codes are less often present ... though engine can start one day running poorer ... no codes present ... and you never know whatta hell is wrong with it ... that's 5s-fe.

[73sport]

Originally Posted by Nervous

How the 5S-Fe determines P0401 / P0402 in this case? It doesn't have this sensor.

Thanks for additional info, btw!

This is a total WAG but the ECU will see changes in both fuel trim and O2 when the EGR is supposed to be active. Couple all that with the MAP and the ECU makes a call. The MZ simply uses more data to make the same determination as best I could tell.

BTW - nice write-up!

[daler]

Thanks much guys for this great thread on the P402 code.

Mine is a '95 Cammy V6 (215k kms). Using the AutoEnginuity scantool I also found the stored P402 code coming on intermittantly.

Interestingly, all the O2 sensors register but the one at the cat does not register on the scantool (laptop monitored). According to the maker, it seems this is "normal" but is a PITA in trying to diag any problems relating either to that O2 sensor or to EGR stuff.

Anyway, after cancelling the MIL it's about another month b4 it comes back on. Never any rough idling, power seems good. But the MIL drives me crazy and the scantool still says everything checks out (once the MIL is cancelled) ..even the EGR test checks as completed.

So it appears that the problem really sporatic. So the question is, how do I best find the offender ..when most of the time the car says everything's ok?

[73sport]

Originally Posted by daler

Thanks much guys for this great thread on the P402 code.

Mine is a '95 Cammy V6 (215k kms). Using the AutoEnginuity scantool I also found the stored P402 code coming on intermittantly.

Interestingly, all the O2 sensors register but the one at the cat does not register on the scantool (laptop monitored). According to the maker, it seems this is "normal" but is a PITA in trying to diag any problems relating either to that O2 sensor or to EGR stuff.

Anyway, after cancelling the MIL it's about another month b4 it comes back on. Never any rough idling, power seems good. But the MIL drives me crazy and the scantool still says everything checks out (once the MIL is cancelled) ..even the EGR test checks as completed.

So it appears that the problem really sporatic. So the question is, how do I best find the offender ..when most of the time the car says everything's ok?

Take a look at this thread. That value at the cat doesn't change much. . Pgs 4 - 5 - 6 have screen shots of software.

P0401 related to Catalyst Monitoring Y/N?

More can be found here. . .
http://s67.photobucket.com/albums/h3...uning%20OBDII/

If you don't find what you're looking for, start a new thread or PM one of us. . .

[rwd4g63]

Hello, I'm new to the site and just wanted to share my findings on these common problem codes. I got a 99 Camry 4 cyl 3 weeks ago but the check engine light turn on after the second time I used the car, had it scanned 401,402,446. For the po446 I went to the junk yard and got the canister with vsvs and all that is attached to it, replaced reset the computer 446 was gone but 401 and 402 kept popping up. cleaned the egr, pipe, did a carbon cleaning kit for 15 min, throttle body but light kept coming back on. I test the egr, modulator and vsv finding the vsv was bad so went back to the junk yard and got me two vsvs but both were bad. I went to the Toyota dealer got the vsv replaced it and the check engine light came back on. found the hoses reversed corrected them and no more problems, idle back to normal. I did get alot of good info from this site that point me in the right direction and got the car fixed cheap plus pass inspection, had a big reject. Thank you all for the great info.

[c32used]

[c32used]

Well I'm back but in this thread...to Nervous after we went back and forth on my thread and I mentioned I found plug to sensor from EGR was unplugged...we as we left off I plugged it in and reset code but it came back on today after a spirited run... but as I looked thru this thread I decided last night after reset to try the breath thru the modular and I was able to breath thru all ports but port "P" was hard but ultimately it flowed...whats the best way to check this? Do I take it off open it remove it's filter and check all ports for obstructions? I gonna try that now anyways and write my results. In the meantime how is it we test the EGR sensor to make sure it works?