I need to know what the following codes mean in order to repair my 2000 Mazda MPV to pass emissions.

P0051:HO2S Heater Control Circuit Low Bank 2 Sensor 1

I know there are two O2 sensors, one for up flow and one for down flow, I need to know which one I need.

Thanks

Bank 2 is the left side of the engine so if front wheel drive it will be the radiator side and if rear wheel drive the drivers side. Sensor 1 is the first o2 sensor closest to the exhaust manifold.

Something definitely related to front side! May be the wheels!