P0420 CODE and O2 sensor conneced?
 

i was having some problems with the rpm's on my 01 ex bouncing every now and then.

the mechanic gave me an estimate of around $800 CAN to replace 2 O2 sensors, and do some tune up work (clean throttle body, change oil, clean in jectors, replace plugs...)

the codes that came up were P0420, P0302, 90303, 90304, 90300.
I Know what the codes mean, but i dont understand how they are related to my O2 sensors. wouldnt changing the plugs, and doing the cleaning be enough? they wanted to replace BOTH O2 sensors... and i didnt think that was necessary. replace it when it's broken was my thinking!

:help: