Just fixed the same problem in my 2003 IS. Was getting p2121 code and limp mode intermittently. First thing I did was thoroughly clean the throttle body wth TB cleaner. It was actually pretty dirty and my car only has 77,000 miles. After reading through forums, a lot of owners recommended switching out the whole throttle body. New ones from Lexus cost about $800. Ebay replicas are about $300. Due to low budget I decided to find someone parting out a IS300 and take a chance with a used one. Paid $170 for the whole throttle body, went and pulled the part off the car myself. It was a 2001 IS300. I swapped out only the app sensor and it has worked great so far. So definitely consider a used one to save some money. Difficulty rating of changing the APP sensor yourself is like 3 out of 10 so definitely do it yourself.