It looks like a failing O2 sensor.
Here's a generic video about O2 sensor replacement.
I replaced all 4 of my O2 sensors as a DIY operation few weeks ago.
Knowing my driving style (6 miles one way to work -- usually 3 stops with 3 cool downs/heat ups) may not be the best for extended O2 sensor life, I like to replace O2 sensors like spark plugs and other maintenance items. My O2 sensors never caused a CEL code but I knew mileage would improve by replacement.
In my case replacing lazy O2 sensors provided the following benefits.
1) Increased MPG - Mileage returned to the level where it was when the Ridgeline was nearly new.
2) Increased pep around town.
I would replace the upstream O2 sensors first.
Check out the following thread.
http://www.ridgelineownersclub.com/forums/showthread.php?t=62274
and the following post.
http://www.ridgelineownersclub.com/forums/showthread.php?t=66074&page=3
NOTE: Do not drop the new O2 sensor. The sensors are FRAGILE.