The downstream sensor is the one that throws that code. But, as weed said, it can only monitor what is in front of it. If there are no detectible leaks, the sensor could be bad or the cat is bad. I had a toyota that was throwing that code and changing the sensor was the fix. But it is important to use the right sensor and one of decent quality. Bosch doesnt have a good rep for toyotas and it isnt well liked for dodges either. If you bought a bosch, the sensor could still be the issue.

 

Nice sig.