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Is this a 4 or 6 cylinder? Auto or manual trans? How many miles on your vehicle and did it fail for a code P0420?
Ok the best place for discounted Honda parts is http://www.hondaautomotiveparts.com/auto/jsp/mws/prddisplay.jsp?inputstate=5&catcgry1=ACCORD&catcgry2=2005&catcgry3=4DR+EX&catcgry4=KA5AT&catcgry5=EXHAUST+PIPE+%28L4%29 which has the part for $443.00. You can get ones on the aftermarket but you will end up with the same problem after a year or so.
To replace the converter you will need to remove the six nuts (three for the front and three for the back), the heat shield bolts and the secondary oxygen sensor. You should spray down the heat shield nuts and the cat nuts to help in breaking them loose. You may also need to heat them. You should get new gaskets for both sides of the cat. When removing the secondary sensor out of the cat converter you should first disconnect it under the front of the passenger seat then pull the connector down through the bottom of the floor. If the sensor is tight you may need an oxygen sensor socket to aid in the removal.