[Idahome]

My check engine light came on today, diagnostic device gave a P0420 Catalyst System Efficiency Below Threshold (Bank #1). I've got 140,000 on the RAV. All written indicators tell me either O2 sensor or Cat, is there a easy/cheap way to determine exactly what it is before I put out the cash to fix it? One thing I've noticed is the fuel efficiency seems to have gone down quite a bit.

Thanks