Hi,
I have exactly this problem. 2 x oxygen sensors and an ECU, £3006 to fit!
My car is a Mk3 GS300 2005 with only 28k on the clock. Apparently the early ECUs had a problem and most were changed under warranty but it seems a few have got through. Give Lexus GB a ring, they should offer 50% towards the cost. I have been lucky enough to get the garage where I bought the car from to pay for the repair as I have only had the car 5 months.
It is absolutely appalling that a known problem like this is is not being rectified free of charge. Lexus cars are not the absolute in reliability.
I think a few more people are going to have the ECU problem soon making a small fortune for Lexus. This is very annoying and coupled with the dealer service being half hearted, my Lexus experience is not a good one.
Do you, by any chance, have any evidence to support the "early ECU's had a problem" comment as I'm sure this would help enormously in my negitiations with Lexus?
Cheers