I have an older Bryant furnace/controller and a Carrier outside unit.
The controller board is a United Electronic Controls Model HK42F2011.

The problem is, the AC will periodically just stop working. No fan, no controller, no nothing.

I checked the voltage to the controller as well as the voltage across the RGYW contacts and they all look OK. The LED indicator light is also on.

What I found out accidentally was when I banged the cover to get it to seat to close, the unit started to work again.
It worked fine all day yesterday (it was in the 90s all day) and then just stopped working in the early evening. So I went back down to the basement and banged the side and it started up again. It ran fine all night.

Any suggestions?
The 3 Amp fuse on the controller is good.
I checked all wiring connections and they all seemed snug. But did not try to reseat all of the connectors on controller yet.

I am reluctant to call in an AC guy because am afraid that when he/she comes the unit will be working or if he/she bangs the side it will start again and he will just check the connections like I did.
Any suggestions of what may be loose or broken?
Any debugging ideas or sequence would be greatly appreciated

Thanks to all!!!

 

Perhaps it's HK42 FZ011 ? If so, that board has a history of bad solder joints which could easily cause your problems. But, before you replace it, you really need to check everything including the solder side of the circuit board.

 

as stated or bad connections somewhere.

 

Thanks for the replies.
Yes it is a HK42 FZ011. (The Z looked like a 2)
Not surprisingly, when I came home from work last night, the AC had stopped working.
As usual, I tried tapping the side of the Furnace with my trusty wood block, but alas, it still did not turn on.
So I took your advice, opened the controller box, tagged all the wires to make sure that I could put it back together correctly and checked all of the solder joints on the PC Board. There did not appear to be any cold solder joints or broken solder connections. All the components seemed to be snug and there were no broken legs on the relays.
I put it back together, re-attached the wires and hooked it back up and made sure that all of the connections were tight, but it still did not work. I tried tap, tapping it again but still no luck.
Later, just for the heck of it, I went down to the basement and tapped it again and, thankfully it started up. Is there a reset switch or a breaker someplace may be tripping?
So, is it the controller board? Should I call an AC repair person? If I call a repair person, what should he/she look for? If it just a board swap, I can do that myself. I do not mind hiring an AC tech to repair it if there was some debugging to do. I just would not want to pay someone just for swapping parts. And to make sure that that the swapped parts are indeed the issue.
Any additional ideas or suggestions would be greatly appreciated.

One other point, several years after a lightning storm, the AC did the same thing. I called a repair person and he just switched the furnace on and off and it came back. There is a lighting arrestor on the outside unit. Just another piece of information that might help resolve the problem.

Thanks for all of your help

 

Hey Pros...what about the safety switch? Not sure how that would affect it...is that just the fan or does it shut the entire thing down?

DIYer...does the outside unit stop as well when the problem happens?

Not saying this is your issue, but...
I had a unit back in VA that the heat was cutting off occasionally. Resetting or tapping around would normally get it working...then it just quit completely. After much searching, it turned out the contacts inside the relays were not always making when energized. Since it was going to be a week before I could get a new control I cut the relays open and cleaned the contacts. Worked like a champ. Couple of hours of work and I knew it should be replaced later. Also...this was a heat issue not A/C but I'm sure it could also happen with the A/C.

 

So, it cooled all night after my tap dance.
But when I got home today after work it was not working.
So I went down and did my tap, tap thing and the fan went on, but not the outside unit.
This is different than before because before the system was completely dead until I tapped it. But at least once it got going, it was cooling. Now, just the fan is working
So the fan is blowing the hot air. But at least it is circulating the air
Any ideas out there?

 

The usual mode of board failure in cooling mode is no power to blower motor. The condensing (outdoor) unit will continue operate.

Now i'm thinking low voltage wiring problem. If you have a meter, see if you get 24 volts ac to the contactor of the condensing unit.

When I suspect low voltage problems, I usually disconnect the thermostat wires at the furnace circuit board and use a jumper to check blower operation. A jumper wire from "R" to "G" should power up the blower motor.

 

Thanks for all of the info so far. I checked the voltage across the terminals and they all were reading about 24 Volts.
And then I got it working. But when I came home from work the fan was blowing but the compressor tripped the circuit breaker.
I reset the breaker and the fan started (on both the air handler and the outside unit) were working but no cold air.
So I finally called the AC repair shop.
The tech came out, checked the voltages across some components on the outside unit.

He said that the compressor was seized because it was drawing 85 amps. The unit is over 20 years old (1989) and he (and others), said it is not worth fixing.

I guess the main question is, if the compressor is drawing 85 amps, fan going no cold air, does that, in fact mean that the compressor is seized?
Is it accurate that, because of the new refrigerant, that they have to replace the internal coils above the furnace?
And 3rd, we had the discussion of whether we should replace the furnace because it is over 15 years old, and that, even though it is still working, that it would be good to be proactive to replace it because of the sheet metal connecting work and the way the new units, with all of the electronics communication and feedback circuits it would better and more efficient and we would have a better overall system if we replaced eh entire system?

 

Yes to all your questions. 85 amps is the "locked rotor" draw...which means its seized. Agree it's not worth fixing. A newer (PROPERLY SIZED and INSTALLED) system will be more efficient and probably keep you more comfortable.

Yes...I'd replace the furnace as well....for all the reasons you stated.

Make sure they actually do the calculations to get the right sized units, not just a rule of thumb estimate. Any improvements in insulation or air sealing should mean a smaller system will do just fine. They also need to take into account duct size and type.