contactor chattering, replaced it- still have chatter
I have a split system (amana condensor RCC36A2E/carrier furnace) and the contactor has started chattering. I replaced the contactor figuring since the old one was pitted that this would fix it, but nope, still chatters. With power to just the thermostat (pulled the outside power), the contactor holds just fine. But once the line power is applied, it will start chattering within a minute.
Have looked over the low voltage line for cuts/nicks/etc, but didn't find anything. Cleaned/scraped and/or replaced the LV connections. Capacitor not bloated.
Can someone offer an idea as to what the problem may be?
Or how to troubleshoot it further ?
My guess is its the 15 yr old control board up in the attic unit (mod# 321257-401). I replaced the control board in the other zone/basement unit a year ago although that was an issue with the heat and I was getting an LED error code. Tested the heat on the attic unit today and it appears to be working fine. No LED codes are showing up on it either. So I hate to spend $150 to replace it if that's not it.
thanks in advance!