Your mechanic should have determined what was wrong before replacing sensors. I assume you've given up on this mechanic.
Are you able to perform any diagnostics or replace sensors yourself? I found the most help was from other knowledgeable owners at Club Lexus forums. I bought an inexpensive code reader from Amazon.com (less than $20) read the codes, posted them online, and was able to determine which sensors to clean, which to replace, and have done the work myself and saved a bucket of money each time. Here is the link to the Club Lexus forums for first generation ('99) RX 300's:
I hope this helps you out. It will only help if you can perform diagnostics and do some simple troubleshooting on your own. After that, the repair/replace of the 14 year old sensor(s) is/are pretty straight forward and usually pretty easy. The parts are readily available. Even if you have a mechanic do the work, you will be better informed.