P0420 Avensis 1.8 T25 2003

[whizzcom]

p0420 Catalyst System Efficiency Below Threshold (Bank 1)

Hi All I got the above code a few weeks ago and changed to front 02 sensor ,code came back.

then changed the 2nd 02 sensor and code came back, the car seems to be running lean ans tick over can be high @ times 1000rpm + ,

could there be a vacuum leak in the system that's causing it to be lean ?

do you think I will need to replace the cat to fix the error code for good ?

if so which cat ?

the front near manifold or the 2nd under the car ?

thanks in advanced :D