Advise re/ Check Engine Light Cat Converter / Sensor $$$ Help

hi, i have a 2003 camry le , 156 k miles and my check engine light has been on for about 6 months .. it is now time to register my car $$$ i have checked a few different muffler shops.. I am being told cat converter and o2 sensor ,, i recently did new spark plugs.. as i was told it was misfiring and had the light turned off it went back on a week later.. unfortunately what i am being told by everyone that i can only get these parts from toyota and they are expensive 2 cat converters $ 400 and $1500 also o2 sensor about $200 tahts over a cpl thousand just for parts.. not to mention my car has a lot of miles... ( did i mention at 99,000 miles my cat converter and o2 sensor went out also ) but was barely under warranty so they honored it.. it just sux that toyota is so expensive and selfish when these common things eventually need to be done for smog / in california ... its outrageous how much... so im kinda lookin into just trading it in/ or selling outright / as/is ???? for a new car .. i think im going honda this time around ...
any advise reguarding my situation / i was thinking maybe first replace the o2 sensor then try to see if it will pass... even though the test is telling me catalytic converter but there are 2 cats which one then will it need or will it be both one on the manifold and one under glove box??? Please help with advise anyone $$$$ my tags are due now????
:crying:

It is not likely that you need a new catalytic convertor. The error code you are getting for a cat convertor is triggered by the rear oxygen (HO2) sensor. Its function is to tell the computer that your cat convertor is bad. However, the computer can't tell the difference between a bad sensor and a bad cat convertor. It is almost always only a bad sensor.

Try replacing the rear HO2 sensor first. Get the DENSO sensors. If you get the OE sensors, they are original equipment and fit perfectly. Go to the DENSO web site, here (http://densoproducts.com/resultsApp.asp), and find the DENSO part number (see below). You can get the DENSO parts in many places on line. I commonly get them from Amazon.com for cheap. Search Amazon.com for "DENSO 234-4048" for example, and the first choice should be the correct part.

Here are the part numbers for your vehicle from the DENSO website. (Note that if you are also going to replace the front sensor, and if you have a 4-cylinder engine, you will have to call a Toyota dealer parts department and give them your VIN number and find out if you have a Calif. emission engine or not).

DENSO part numbers:
Front sensor 4 cyl Calif. 234-4622
Front sensor 4 cyl non-Calif. 234-9010
Rear sensor 4 cyl all 234-4048

For the above rear sensor: See here (http://www.amazon.com/Denso-234-4048-Oxygen-Sensor-Exhaust/dp/B000C5SG1I/ref=sr_1_1?s=automotive&ie=UTF8&qid=1310313205&sr=1-1)

Front sensor 6 cyl all 234-9047
Rear sensor 6 cyl Left all 234-4261
Rear sensor 6 cyl Right all 234-4064

For the above sensors, see here (http://www.amazon.com/Denso-234-4261-Oxygen-Sensor-Ratio/dp/B0016NBBQW/ref=sr_1_1?s=automotive&ie=UTF8&qid=1310313403&sr=1-1) and here (http://www.amazon.com/Denso-234-4064-Oxygen-Sensor-Ratio/dp/B000IYBFE8/ref=sr_1_1?s=automotive&ie=UTF8&qid=1310313567&sr=1-1)

Here is the DENSO web site URL:

http://densoproducts.com/resultsApp.asp

1) I would consider HO2 simulators for $35 down stream to supress the DTC's P0420 & P0430.
2) A good DIY tuneup.
3) Once you weed the problems out with a good tuneup, see http://www.catalyticconverterwarehouse.com for new Cats.

It's funny in Cal you can smoke, but your cars can't!

Replace the sensor first. Then worry about a cat convertor.