I have Goodman inside and outside unit that's controlled with a 3-zone Honeywell zone controller. For the last three months our AC has been very intermittent and will work fine for 3-7 days and then all of a sudden not work for 3-15 hours before magically working fine again. Often it won't be working when we go to bed but will turn back on sometime in the middle of the night and sometimes visa versa. Sometimes I could get it working again by pressing the reset button on the ZC board, sometimes by resetting the power and once even by hitting the unit the back of my fist (and using foul language.. hey, it was 2am!).

I've had four different HVAC companies out that end up scratching their heads or leaving once they find out it's a zoned system, but they have all checked Freon levels and all said the system wasn't freezing up. The latest tech thought for sure it was the 8 year old Honeywell EMM-3U ZC board's relays getting confused and hangin' up. He gave me an estimate of $750 to replace it with a generic board that he said would probably break within a year, or $2800 for an entire new zone control system (board, dampeners, etc).

So instead of paying for either ridiculous estimate, yesterday I replaced it with Honeywell's new HZ322 direct replacement model for $150. I'm not an HVAC specialist but the retrofit was extremely easy and I'm very confident it's hooked up properly. Well immediately after I hook power back up it fires right up! YES! I thought. Well it turns off a couple minutes later.. So I hit the purge override and it turns back on! Then cuts back off a few minutes later. I played with some of the settings and changed the heating fan to be controlled by the board instead of the HVAC and it seemed to work fine for the remainder of the day, till about 5am. Now the unit hasn't been on since. It won't respond to any of my three t-stat calls. The HZ322's on-board diagnostics have an option of turning the individual components of the HVAC system on and off and it's not responding to any of those requests.

So now what? I have a basic multimeter, but not sure where to start probing around. I've been able to eliminate the t-stats and ZC board from the issue now. Does this sound like a bad furnace board? Also of interesting note: the outside unit and inside fan never come on separately, they both only come on or stay off at the same time. So when I put a call in from any t-stat one of the two never come on by themselves—always together or nothing.

Problem solved! I was playing around testing voltage and discovered no 24v on the red and white equipment wires coming off the ZC board while one or more tstats were calling for cooling. I figured the ZC board should have been supplying that voltage and decided to hook one of the tstats directly up to the furnace board to completely bypass the ZC board. I removed the furnace door and taped the door switch up to stay on while I tested a few more things. I didn't find much until I started to follow the white common wire up to the furnace board and while doing so it jiggled a bit and BAM, furnace board LED turned on, blower started up and the outdoor unit turned on! Turns out the prong on the board is week and looses connection frequently, requiring me to take the door back off, jiggle it around and the unit comes back on 100% of the time. I removed the board to look for solder cracks and it looks fine so something must be messed up beyond what I can see. Time for a new board! This would explain why the blower and outside unit would always turn on and off at the same time. I didn't realize the 24volts were supplied from the furnace boards transformer--I assumed it was supplied by the ZC board's power.

Its bitter sweet that it was something so simple but it really pisses me off that four different AC techs couldn't figure it out and instead insisted on spending $750-$2800 that only needed a $75 furnace board in the first place. Not to mention my home warranty denying the repair because of a misdiagnosis from their assigned tech that took two steps in my front door, threw his hands up and turned around and left when I asked if he could work on a zoned system.

Case closed, thanks to everyone for looking!