cel came back on

[jd1828]

i had a cel on my car for a while now. it was a p505 which is something to do with the idle control. i assume that it was a bad isc so i picked up a used one. i was told the used isc was still good. i put it on a few days ago and cleared the light. almost two days later it came back. i checked the code and its the same p505 code. is there something else that i should fix. the idle use to bounce around from 1k to 3k really fast and would die all the time. i havent had that problem since changing it but the light did come back.