02 chevy blazer, P0155 error code

I have a 2003 chevy blazer 4x4 4.3lt v6, I am getting error code P0155, does it meen the RT bank upstream o2 sensore is not working, and is that the one between the rt exaust manifold and the cat?
Thanks

That code means the heater is not working on that O2 sensor. It could be a wiring issue or a bad heater in the O2 sensor. That should be the first sensor on the right bank (right from drivers perspective).