Fixing P2631 in a 2006 Honda Accord

This code is "air/fuel sensor" bank 2 sensor 1.

The vehicle has several such sensors, with this stupid bank nomenclature. Since it is a V6, there are 2 "upstream" sensors, and at least one "downstream" sensors. The upstream sensors are right on the exhaust manifold. In fact this one is front and center in the most convenient position possible. The other upstream sensor is for the other 3 sensors and is less handy back by the firewall. The downstream sensor is under the vehicle near the cat convertor.

Note that there are both oxygen sensors and air/fuel sensors. The downstream is/are "oxygen" sensors, and the two upstream are "air/fuel". The difference is important. An oxygen sensor just gives a "good/bad" signal, maybe a 3 way signal -- too rich or too lean but not how much so. An air/fuel sensor gives some kind of analog value that conveys much more information. And air/fuel sensors are correspondingly more expensive.

Rock Auto would sell me an NTM upstream sensor for $110.

My vehicle had been parked nearly a month. When I started the engine, the CEL came on, and I read his one code. I made a visual inspection expecting packrat chewed wires, but found none. I then drove the vehicle about 5 miles and the CEL light went out!!

So nothing to do now, but clear the code and figure that this was somehow related to the vehicle sitting for so long.