Most of the time when a Honda idles like that, it's more than likely the O2 sensor has failed. The first to do is to take a needle and backprobe it into the wire. Take a multimeter (I have a snap on vantage pro for graphing purposes) and while still plugged in, watch the voltage. A good working O2 sensor still plugged in is gonna try to produce any where from .300 to .700 due to the computer doing the rich/lean switch (basicly for emissions) and you have to be certain that it's switching in that manner to make sure it's not stuck reading too rich or too lean. Another thing you can try is the bench test. Unscrew it and with heat put a bottle of butane on it and be sure to move it around the tip in a circular manner or your destroy it as it will surely burn a hole through the insulator. The O2 sensor should produce when fully hot 1.27 volts and when you take the heat away from it it should drop like a rock. Anything under that and it's no good. Try that and see if it works.