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Q: P0401 - have followed posted help - still have problem. on 1999 Toyota Camry

Am getting the engine light about once a week (I reset it - it will stay off a while then come back on). It always reports P0401 Low EGR flow. This is the Japam made camry for those that know the differences.
I replaced the EGR valve and gaskets - verified the ports were not clogged (used pipe cleaners and ran them through until they were sticking inside the manifold). I replaced the diaphram filter thing next to the EGR with the hoses interconnecting between them - all hoses are clear and I used only real Toyota parts.
I checked the VSV valve and I think it's acting abnormally. After warming up the engine - I can quickly increase rpms to 3000 and I notice the EGR valve opens for just a split second and then closes - even if I hold the RPMS. It does this repeatedly. I found that the VSV valve is causing it - opening the air flow just for a second, then blocking it again. So - there's no situation when the EGR valve stays open - even at 3000 rpms. Maybe this is normal for this car, but maybe it's not - any ideas? I bought the vsv valve too when I got the other parts but I hadn't put it in yet because it was so hard to get to. Any suggestions on how to get to that valve are welcome too. I did the vacuum drop test recommended by an earlier post (connected a vacuum guage to the manifold - was reading pretty high - around 24, then I manually opened the EGR valve with the engine holding at 3000 rpms, The vacuum did drop 3 on the gauge.)
One note: I got a MIL error saying the front O2 sensor heater had failed before I had the problem with the EGR error. I replaced that O2 sensor (also with geniune Toyota part) and it wasn't a week later I got the first EGR P0401 error. Could be bad luck - but if I got a flakey 02 sensor - how would I know? Does anyone know how to test one using a multimeter and what readings I should look for?

Please advise on the EGR VSV valve suspect problem (is it working normally but turning off right after turning on?

Please advise on best way to get at the VSV valve to change it. (can't seem to reach it front the top or bottom.

Please advise on how to test the O2 sensor when I have only a multimeter. I have an electronics background - I just don't have a Toyota tester.

Thank you all very much for any advise - this has been a frustrating problem. I will do what it takes to fix it but I don't want to keep buying parts that don't solve the problem. The car is a 4 cylinder automatic and has 70k miles on it.
Respectfully, Dennis
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The VSV on Toyotas do go out. I have replaced several. Here is an article on P0401-Toyota I wrote for RepairPal, as there are many People and Techs that get stumped on this one. ( I do Emissions work in CA)
Since you did not say that you have a 6 or a 4 cyl, if it is a V6, some of those have an EGR temp sensor. If so, be sure to test that. A DVOM and a good vacuum gauge and some patience is all you need to test this system.
As for the EGR P0401 coming on after the new O2 was installed, the Monitors run in a sequence, so if you have an O2 code, the engine computer ignores the EGR system.
see the series of Monitor articles I wrote, because this too stumps People and Techs.
My Camry 99 with 4 cyl got error code p0401. I had it changed egr valve, egr solenoid and modulator. Still shows the code with 401 and 402 with fluctuating idle when engine is warm.

Is the egr solenoid the same as vacuum switching valve? Where does the egr solenoid located?
Like Dan, I have seen the Vacuum Switching valves on the Camry test good and fail intermittently and cause a Check Engine Light to come on signifying low EGR flow.
Additional Information My 99 Camry is a standard 4 cylinder Japan built.
I did a test today - bypassed the VSV with a section of vacuum hose and started the engine - the EGR did work normally and when I held the throttle at 2500rpms, the EGR stayed open tapering closed as I slowed back down the RPMs. I put the VSV back in line - the EGR opened quickly, then closed again and stayed closed. It acts like the little relay in the VSV starts to open up but then the circuit itself inside the VSV opens and that closes the VSV. So to me, that bypass test narrows the problem to the VSV or it's supporting circuit.

One question wasn't addressed - am hoping for advice on how best to change the VSV valve. It seems to be in an impossible to reach location on the back on the engine below the manifold. When I crawled under the car, it seemed to be above the motor mount and I couldn't see it or get my hand in there to feel it either.

Thanks you Dan and Pat for answering - I hoped one or both of you would weigh in on this one. Now - can you tell me how best to get that VSV valve replaced.

Thanks again - very much!
Getting to the VSV to replace it is not easy. I found that if I took the brace off that goes between the block and the intake manifold, and then unplugged the wire to a sensor on the back of the block, I could then reach up in there and get the bolt out that holds the VSV bracket. Once I got it unbolted, I could pull it over enough to be able to get into a position that I could unplug the wire and vacuum hoses from the VSV.
Just read today 2/17/2016 have same problem. Put car on ramps reach up under plenum and unbolt vsv , soaked in wd lube for 30 + mins. , no help . But But But moved vsv to end of plenum rite behind oil fill cap . small 4 mm hole??? , find bolt to fit and moved it there. Now it is rite there on top and accessible. eletric connector reaches and hoses do also. used tie strap to secure vac hoses to fire wall fittings back there. Repair in progress , will change vsv and see if code goes away.
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