According to this it's part #1533 for the passenger side and #1534 for the driver's side.
http://lexus.sewellparts.com/oem-catalog/1104116296-EngineAndFuel-LS400-1999.html
There is also a screen filter for the OCV's part #15846A Perhaps this has become clogged and needs cleaning.
PS: If you haven't reset the ECU by this point then you could very well be barking up the wrong tree.
Sorry for the late reply. I don't drive my IS very much. I only just turned 35K miles on it. I've had all the recalls done although I never noticed any of the symptoms Lexus mentioned. Car runs as good if not better than when it was new.
With the odometer in total mileage mode, press and hold the reset button down. While holding the button, push the car start button and watch the numbers on the odometer. It should "spin" for a bit and then show 0000. That's the reset.
Go to lexus.com and register your VIN. You will then be able to access the owners and maintenance interval manuals. I think you can also access your vehicle's maintenance history.
can you send me any assembly or breakdown pdf's for the 2001 IS300? I found an old post of yours from 2008 for the door assembly that helped a ton! thanks! wondering if you have any more spec sheets to offer