I am reading all of the posts discussing the possible reasons for P0401. i am getting the code consistently on my 97 lx450 after the vehicle warms up and I am cruising along at 2000-3000 rpms. I see that a lot of people fix the problem when replacing the VSV. A mechanic told me he did a test and it appears to be my EGR valve (but he is not a Lexus specialist). After studying the function of the VSV, and seeing that it is either on or off, I figured I could test whether or not it was functioning by plugging the line between the VSV and the EGR, simulating a positive condition. (my resource indicated that a negative condition would provide an atmospheric bleed, disabling the EGR). If the function of the VSV is to allow the vacuum from the modulator to activate the EGR, then my test makes sense, but if the VSV provides a vacuum itself, it is not a good test. Does anyone know whether or not the VSV provides a vacuum? By the way, after pluggint the line from the EGr to the VSV, I still got the code, which would indicate it is not the VSV.
Thoughts?