Hi Jaspersilus,
I've had the same problem. I changed to 2 A/F sensors, pulled and replace the ecu fuse and the lights were gone. Two weeks later the lights are back on. A diagnostic check shows up ' Fault Code P0420 -
Catalyst System efficiency below threshold (bank 1)'.
I have looked up Lexus rx300 diagnostic Fault Code List on internet and that code shows trouble area
1. 3 way cat. converter.
2. Open or short in Oxygen sensor (Bank 1 sensor 2) circuit.
3. Oxygen sensor (Bank 1 Sensor 2).
4. Open or short in A/F sensor (bank 1, 2 sensor 1 ) circuit.
5. A/F sensor (Bank 1, 2 sensor 1).
Now I'm lost.
Hope you get your Lexus sorted.
Cheers
dad1.