Troubleshooting a Two Zone Furnace controller

AprilAir Two Zone controller I found bad, shorted out call for heat to the furnace, furnace check out okay. Both zone controller check out okay. Did some poking around the Two Zone controller and it came back to life and worked for a bit then stopped, Cold solder joint? ESD? I will never know.....it was replaced by another technician. Oh well, I hate this "system" troubleshooting crap.....replacing the whole board for one bad part, I miss going down to the component level sometimes.