Been having issues with diagnosing a trouble code involving the EGR System on my 97 Previa, and wondering if anyone out there has had a similar issure or any ideas. The DTC is #P0401, "Exhaust Gas Recirculation Flow Insufficien Detected".

It first came on last fall during a road trip in the mountains, and the mechanic i took it to thought it might be caused by the high elevation. They reset it, and it stayed off until early this summer. Since it came on again, i've been intermittently trying to fix it. Seems to be running fine, and the gas milage is as good as its ever been ( i've read that low mpg's would be a symptom?).

My first step in diagnosing it was the following the system check in my Chilton's manual on pg. 4-13. I checked the Vacuum Modulator filter and it was filthy, so i replaced the whole part. Reset the code (by disconnecting the battery), but the check engine light came on again within an hour of driving.

Next i checked the egr parts using a vacuum gauge as outlined in the system check ( same as the one in the Toyota Repair manual on pg # EG-125). However i dont have a ODBII scan tool or a tachometer, so i guestamated the rpms through the process. They all seemed to be fine until checking the EGR Valve. The engine was supposed to die after bridging the EGR valve to the intake manifold, but after a couple minutes was still running fine. I then took the valve off, cleaned it, reinstalled it, and drove it for a while, but the code came up again. I thought i found the problem and bought a new EGR valve for about $150. Reset the code, drove it, but it still came up again.

Since then i bought the toyota repair manual and have been going through the steps outlined for the P0401 code on pg. EG-279. Checked the gas temp. sensor while installed, took it off and did the heated oil tests and the resistance values were pretty close.the harness was also good. Connection of the EGR vacuum hoses also checked out. Next i looked at the VSV for the egr. Didn't mess with its operation thru the ECM ( was warned its very easy to break), but followed the steps on pg. EG-127. It checked out good for both an open circuit and ground. Blowing thru pipe E I couldn't really feel it at the air filter, but could hear air escaping somewhere. Next i hooked it up to the van's battery. i couldn't get it to blow any air out, it would just make a clicking sound when the wires touched its terminals. So now i'm figuring that the VSV is bad? Checked with my local Toyota dealership and they can get a VSV for about $140ish.

So before I buy another part i don't need, I'm wondering if anyone thinks my diagnosis is correct or if i'm missing something here. Also, wondering if there's any possible bad results from continuing to drive it while the problem exist? And sorry for such a long question, thanks!