no ecm reflash is needed, i replaced mine a couple of thousand miles ago, if ones bad, its best to replace both at the same time.
i bought 2 Bosch o2 sensors off amazon for i think 85 or 90$
here are the exact ones i bought
http://www.amazon.com/Bosch-15...r=8-1
just checked the boxes in the garage.
then all you have to do is splice 3 wires from the old connection underneath, theres a mini waterproof box included in this and they work great.
They work great and are much cheaper then anywhere else will charge you.