craftsman limit switch

i have a 2008cfaftsman door opener.  the other day the door opened but didn't stop when it was supposed to and the chain got in a bind.  i got it loos and through research was told it could be the rpm sensor.  replaced that and now the up sensor works correctly but the down sensor does not stop the door.  it goes all the way to the stop bolt.  i have another craftsman opener on my other door, which works fine.  i swapped limit switches and get same thing.  is my limit switch bad or could it be the logic board?  the cheapest board i find is $125, so it makes no sense repalcing that, since i can buy a new opener for $170.  any suggestions appreciated.