2003 GMC Sonoma Serviice engine light. Have code.

Yesterday my Service Engine Soon light came on. No drivability issues and everything seemed normal. I wasn't too far from home but made a couple more stops on the way home. After the second stop I came out and as I went to get into the truck I smelled coolant. Then I went up front and looked down to find a puddle of coolant right underneath the cross member. I knew is was coolant and not A/C drainage. I was only a couple miles from home and the temp showed pretty much normal so I drove home.

After looking at things as I suspected the water pump had failed so I headed to the parts store to buy a replacement. I decided to purchase an OBD2 code scanner also to find out what the code was. I got the water pump replaced and things seem fine and I hooked up the scanner and this is what I got.

P0155 02 heater circuit
HO2S-21 (Bank 2 Sensor 1) Heater Circuit Malfunction

It appears to maybe be an O2 sensor. If so, which one and where is it on this vehicle. Any help would be greatly appreciated.

Thanks!

John