I have discovered a possible solution to chronic CEL and thought I would pass this on, but would also be interested to hear from anyone if this mechanically makes logical sense...
My '99 RX300 has 100,000 miles and I've replaced all the O2 sensors because of CEL codes (even though my car seems to run just fine). I live out in the far suburbs and my commute everyday is only 7 miles one-way on back country roads at 35-45 mph. Maybe once or twice a month do I get on a freeway to run an errand and drive 60+ mph. I noticed that every CEL came while driving at low speeds with RPM under 2000. The thought occured to me that maybe my engine needs to run between 2000 - 3000 rpm a little more often. About 3 months ago, I started driving with the Overdrive off...and then I only engage it if/when I reach 50 mph(approx 3000 rpm). Yes, my gas mileage has dipped from 18.5 to 17...however, I have had no CEL's and the engine seems to be running very well.
Does this make sense to anyone?
Thx!

 

If the torque converter would lock in with out being in OD,I would do that more too.But the engine speed is too high to suit me.

 

What was the CEL code(s) in question that you got after replacing the sensors?

 

I was getting P0171 all the time, but from I've read from your posts, that's a common code.

 

MY RX is now in the 135K range or so and I get these codes every so often. Granted my RX has modifications to it, but it's aging just like any other vehicle and having the original A/F sensors, kinks are bound to occur.

 

My RX is over 165k now and have no code. I don't think it is normal having CEL on at lower speed with OD on. Mechanically, your system is faulty and will suggest you get someone competent to carry out the diagnosis.

 

The only thing other that I can think of is cleaning of your MAF and IACV. In addition, sometimes the weather at certain times of the year (incoming air temperature) can cause issues.