The ls400 is a very complex car when it comes to reading faults.
The number of ECUs is too much for generic code readers. However, you can get Toyota TIS Techstream mini VCi rather cheap on ebay and that works perfectly.
Alternatively you can get the codes using the TCCS method. Bridging 2 pins on the connector and read the codes on the dash. On the earlier models this is achieved by counting the times a light flashes, on the mk4 you can get the actual codes displayed on the multi info display by pressing the 'function' button.