Po171 error code on my new(ish) Lexus RX300.  It flags up OXYGEN SENSOR 1 BANK 1.  Already replaced in January, I had it replaced again for free.

CEL / Engine light comes on after 100 miles or so.

More investigation – Lexus just guess, no concrete ideas.

So I go to www.clublexus.com and get some tips from the very helpful users

A/F sensor (the one behind the engine)
MAF sensor
Faulty fuel cap 
air intake (hose loose)
fuel line pressure
injector blockage
engine coolant temperature sensor
gas leakage on exhaust system

 I cleaned the MAF 1st time, no difference.  Cleaned the MAF again with contact cleaner and a cotton bud.  Disconnected the battery before hand of course and that reset the CEL / Engine light.

I’ve plugged it all back in and going to run it to see what happens….