buffalo

I've been driving Lexus since 1990 and never had a V V T I sensor light. Did not know what it was. Oxygen sensors go out frequently, much to my dismay, but not VVTI. Currently I own a 2002 LS 430 and need to replace the VVTI. The dealer says it is under the valve cover and will cost $13 million dollars to repair it. The O2 sensor is even more but I know they are easy to get to and replace. How does one replace the V V T I sensor?