So I have a P0420 and I want to replace the downstream O2 sensor. I have a new A/F ratio sensor and I'm pretty sure my cat isn't bad so I'm going to go ahead and change that downstream O2 sensor and hope it solves my problem. My question pertains to the actual part. I called Toyota to ask them how much the part was and if I could get a part number. After I gave them my VIN number they told me that my 2000 4runner Limited is a California emissions (I live in NY). They didn't have the part in stock and would have to order it ($170). Toyota uses Denso sensors so I know I could get this same sensor for at least $70 cheaper. Does anyone know where I could purchase this sensor online? I don't want to get the wrong sensor considering I have CA emissions and what not. The part number Toyota gave me is useless because its a Toyota part number; not a universal part number. Thanks.

I think almost all 4Runners starting that year adopted Cali standards. The downstream is actually cheap.

Look for the part number here according to what your 4R has:

Oxygen / AFR Sensors - Underdog Racing Development

Then order the part either from Amazon or Rock Auto.

Last thing you need to do is clean the MAF sensor, both wires.

I HAD a P0420 code too with an emissions test required.

I replaced both upstream and downstream Denso sensors from NAPA.

Do both so you really know.

Code gone.

Look into RockAuto Parts Catalog. You can get Denso 02 sensors under $50 each. I'm waiting for payday to replace both my 02 sensor. Hope it helps.