It should be teh sensor in the exhaust manifold. I dont know for sure, 'cause I only hae one of them. You dont necessarily need to replace the sensor. The code says that its the malfunction of the heater circuit. Search here for that, theres info on how to test the O2 sensor.

There are a variety of different o2 sensors that you can buy, ranging form universal to OEM. Most people would not recommend universal. I went with a Denso, which worked fine for me.