I read some posts on here I have a 1998 LS400 with 278,000 miles check engine light came on and it said inefficient catalyst bank 2. I changed all 4 O2 sensors about 2 weeks later light came on again inefficient catalyst bank 2 so I bought new converters changed them out the light stayed off about 4 months. Now it keeps coming on but this time it's bank 1. I thought maybe it might be a leaking gasket before the post converter O2 sensors so I replaced them it's still coming on showing bank 1. I replaced the mass air sensor because the exhaust smelled a little rich but no change in that, still showing bank 1. I hope some one knows what to do I am a good mechanic but this has got me pulling my hair out.

 

Check the ignition coils. It sounds like unburned fuel is getting into the catalyst.

 

What MPG is the car getting now?

 

if you pulled your plugs you could tell which one was bad

 

About 20.5 to 21 mpg it has gotten better than that I live in a rural area with very light traffic or no traffic at all it has gotten 23.5 before

 

I will check the coils and change the plugs thanks I will let you know the results