BlackSpade

Just an FYI, I had the same problem with my 99 RX 300. I anticipated having to replace the knock sensors and other parts. Instead, after a little testing (moving the coils around and seeing which cylinder was misfiring) I was able to narrow it down and I replaced a SINGLE ignition coil pack and all the problems went away. Ilding became smooth, and the overdrive started to work again. It seems the RX300 can have a coil pack fail, which then causes that cylinder to miss. The resultant vibration sets off the knock sensor (and corresponding code). The activation of the knock sensor then sets off the Lexus "limp home mode", which prevents overdrive from engaging. You could chase knock sensors or other repairs, but in my case I was able to determine it was just one failed coil pack. Once that was replaced, the codes were cleared, and the car now runs fine 100%. Be double sure that before you go replacing knock sensors that you first deliberately rule out that you don't just have a failed coil pack causing the failure and somehow triggering the ultra sensitive knock sensor. If you don't get to the source of the problem, then just replacing the knock sensor won't solve your problem. I said be DOUBLE SURE that you don't just have a failed coil pack and here's why: I initially had the coils individually tested, and the (voltage) test showed they were fine. But the test itself was faulty. Had I listened to that mechanic, I would have replaced a whole bunch of parts rather than just the one coil. Instead of accepting his diagnosis, I asked him to move the coils around one at a time. Each time, I would check the codes to see if it was, say, cylinder no.1 or no.3 or whatever. Within a few tries I narrowed it down to the exact cylinder no., and then replaced just that one coil. Started it back up and it was smooth as butter again. So don't just rely on a voltage/amp test. Actually physically move the coil to another cylinder, and see if that cylinder acts up. And also: You can get very low prices at CarQuest, lower than just about any other place I've found including Advance Auto and other places. I think I got mine for like $49, after getting initial prices of over $75 on Amazon and Advance Auto Parts.