Repairing a HK42FZ007 Furnace Control Board

PLEASE DO NOT ATTEMPT THIS IF DO NOT KNOW WHAT YOU'RE DOING. Your furnace control system is a critical safety system. If you repair or install it incorrectly or damage the system, it can burn your home down or cause a gas leak with people inside, causing severe injury and/or death. I cannot emphasize this enough. Be careful, PLEASE! The following information is for informational purposes only.

For those people who would like to replace rather than repair their furnace board, I believe something like this should work: HK42FZ007.

The non-condensing forced air gas furnace in our home had been having intermittent trouble starting up. The furnace brand seems to be "Night and Day" and the model is Plus 80. The normal startup sequence is something like this:

1. Inducer fan starts
2. Hot surface igniter glows
3. Gas turns on
4. House blower turns on

The problem we had was the inducer fan in our furnace would cut in and out between step 1 and step 4. Most often, it would cut out right as the gas valve opened. This would correctly cause the inducer pressure switch to shutdown the furnace (the pressure switch is a safety mechanism). The problem here is that the inducer was cutting in and out; the inducer should not cut in and out if the thermostat is calling for heat.

After doing some initial debugging and internet research, I learned that our furnace uses a Carrier HK42FZ007 Furnace Control Board. This board is similar to a number of control boards such as HK42FZ004, HK42FZ008, HK42FZ009, and HK42FZ011. From my internet research, it seems that all these boards except the newest HK42FZ011 have a design flaw that causes one or two of the power resistor solder points to crack, leading to an intermittent connection. The symptoms are intermitted inducer behavior much like what our furnace was doing.

I pulled the board and took a peek. Here are some photos from this project:

The control board inside the furnace, showing wires connection points.

The control board inside the furnace, more wires.

The control board inside the furnace, even more wires.

To remove the board, there were two sheet metal screws holding the black case to the furnace. Make sure to record which wires go to which terminals before removing the board; I took detailed photos above.

The control board is out of the furnace.

Closeup of the control board box.

Closeup of the control board box another angle.

Back of the control board box.

To remove the board from the case, gently pry the back off. There are clips all around the edge, and if you are careful, you can remove the back without damaging the case. Inside, you will find the circuit board:

Circuit board front. Click for a larger version.

Circuit board back. Click for a larger version.

After some close inspection, I found cracks in the solder on R33 and R34, the large resistors in the board's cutout. Here's a closeup of the cracking. You can see the cracks on the two solder points on the left side of the image:

Cracked solder on the left two solder points.

To repair this problem, I used my soldering iron and reflowed the four solder points on those two resistors and added some more solder to the points also. Here's a photo of the repaired board. It's hard to see but the solder points are nice and solid now.

Repaired solder cracks.

I reinstalled the board and ran the control board's self-test. Everything seemed ok, so I fired up the furnace, and I have not heard the inducer hiccup for more than 6 hours now. I'm pretty certain the problem is fixed now.

I hope this is helpful for people out there facing a similar problem.