Toyota Repair: Check Engine Light, 2002 toyota camry, catalyic converter

Question
I currently have a 2002 Toyota Camry LE and the check engine light just came on two weeks ago.
I had a diagnostic test and the code received was "P0420", O2 Sensor.
The service station gave me a quote of $800 to fix the sensor.
However when I contacted a Toyota dealer, they stated for that price, it sounds more like the catalyic converter.
Which is true?
I have yet to have my 90K maintenance on the car, and the service station said the check engine light was not caused due to the timing belt, but due to Toyota warranty, the timing belt could go at no moments notice.
Also, the service station advised due to this issue, I would fail emissions, however my emissions passed in April.
What is P0420 code and is it actually the O2 sensor or could it be the catalyic converter?

Answer
The PO 420 code is not an O2 sensor failure code, there is a problem with the catalytic converter, if you reside in the US the warranty for the converter is covered under the federal emission control warranty for 8yrs/80,000 miles, also you didn't say whether it is a 4 cylinder or V-6 engine but if it is a four cylinder it has no timing belt, if it's a V-6 it does and it needs to be replaced at 90,000 miles.