1999 toyota avalon xls with bad O2 sensor p/n 8946741021 where is it?

Question

I was told that my O2 sensor p/n 8946741021 is defective and would cost $350. to replace. Where is this sensor located and can I use an after market part instead of a Toyota sensor? Is this the O2 sensor in the exhaust manifold nearest the radiator or is it in the manifold nearest the firewall or perhaps located after the catalytic converter? Can O2 sensors be cleaned like the MAF sensor and be a worthwhile repair? What should a new after market O2 sensor cost and what is a very good supplier? I don't know the actual code, was told this was the defective part along with a defective MAF sensor which I know it's location and how to clean.
This is for a 1999 Toyota Avalon xls with a 3.6 liter V6 engine with an actual 26,000 miles use.

Answer 1

P/N 8946741021 This is for the front 02 sensor, you have 2 front 02 sensor, the 02 before the catalytic is the upstream 02 sensor which are the engine management sensor and the the 02 sensor after the catalytic are the downstream 02 sensor or the catalytic monitor sensor. Depending on the code that you have there would be a code for Bank 1 or a code for Bank 2. . If you have a code for the MAF sensor you need to check to see if there is any exhaust leak prior to the front 02 sensor, that exhaust leak just prior to the front upstream 02 sensor will set a MAF code and also a slight rough idle.

 Here a Toyota, this vehicle set a front bank 1  02 heater circuit code, and a MAF circuit range / perfromance code. having a exhaust leak just prior to the 02 sensor, the exhaust leak was fixed , just resolving the exhaust  leak solved both code condtion.