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:
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.
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!