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.

How To Change the Struts and Strut Mounts on your 1998 Toyota Camry

PLEASE DO NOT ATTEMPT THIS IF DO NOT KNOW WHAT YOU'RE DOING. Your car's suspension is a critical safety component. If you mess up, your car will become dangerous and deadly to you and everyone around you. The following information is for informational purposes only.

This blog post describes how to replace the struts and strut mounts on a 1998 Toyota Camry yourself, but it should apply for Camrys from 1991 to 2001, and probably other models as well. This is not a difficult project, but it does take some time. It took me about half a day to do everything.

To complete this project, you will need an assortment of sockets, wrenches, a breaker bar, a torque wrench, spring compressors, and an allen key.

Struts generally go bad gradually, and at higher mileage. This car has 120k miles, and the struts were leaking oil. The strut mounts in this generation Camry are a known weak point. The symptoms of bad strut mounts are rattling over minor bumps, particular when the car is cold.

I picked up the parts from autopartsarehouse.com. They have pretty good prices for the struts and the strut mounts; I got the KYB brand. The bellows didn't fit though, so I reused the old bellows.

Section 1: Changing the front struts

Step 1: This is the front strut assembly in the car.

Step 2: Remove brake line, unclip ABS sensor wire clip, spray PBlaster on sway-bar endlink nut.

Step 3: Undo the two strut-to-knuckle bolts.

Step 4: Undo the sway-bar endlink nut with an allen key and wrench.

Step 5: Undo the sway-bar endlink nut, done.

Step 6: Undo the three nuts holding the strutmount to the strut tower.

Step 7: Support the brake rotor/knuckle and remove the strut assembly from the car.

Step 8: Close-up of the strut mount and spring seat.

Step 9: Compress the spring with spring compressors.

Step 10: Compress the spring with spring compressors, done.

Step 11: Remove the top strut nut; be careful!

Step 12: Remove the top strut nut; be careful!

Step 13: Remove the strut mount.

Step 14: Separate the strut parts: strut, bump stop, spring seat, spring, bellows, sprint seat.

Step 15: Reassemble the strut assembly with the new strut and strut mount. Reinstall the top strut nut.

Step 16: Make sure to use the correct strut since all 4 are different. Also make sure to orient the strut mount such that the arrow is pointing to the outside of the car.

Step 17: Complete strut assembly with the new strut and strut mount.

Step 18: Insert the strut back into the strut tower, and loosely thread on one of the three nuts to support the strut.

Step 19: Complete strut assembly with the new strut and strut mount.

Step 20: Line up the two strut-to-knuckle holes with a prybar or extension bar and insert the bolts.

Step 21: Thread the nuts onto the strut-to-knuckle bolts, but do not tighten them yet.

Step 22: Tighten the three strut mount nuts to the proper torque specification.

Step 23: Reinstall the sway bar end link to the strut and tighten the nut.

Step 24: Tighten the two strut-to-knuckle bolts to the proper torque specification.

Step 25: Tighten the strut top nut to the proper torque specification.

Section 2: Changing the rear struts

Step 26: Rear strut overview, support the brake drum.

Step 27: I don't remember the details here, but I believe you need to: 1) Fold the seats down, 2) Remove the interior panels on the c-pillars, 3) Tug forward on the rear package shelf to remove it.

Step 28: Remove the seat belt tensioner.

Step 29: Remove the bracket for the brake line.

Step 30: Spray PBlaster on the sway bar end link, and remove the nut.

Step 31: Spray PBlaster on the sway bar end link, and remove the nut, done.

Step 32: Undo the two strut-to-knuckle nuts.

Step 33: Undo the three nuts holding the strut mount to the body.

Step 34: Remove the strut from the car.

Step 35: Compress the string, and undo the top strut nut.

Step 36: Disassemble the strut.

Step 37: Reassemble the strut with the new strut and strut mount. Install the strut top nut.

Step 38: Reassemble the strut with the new strut and strut mount. Install the strut top nut, done.

Step 39: Like the front, hang the strut by one of the three mount nuts.

Step 40: Reinstall the two strut-to-knuckle bolts, sway-bar end link, and brake line. Use the correct torque specifications.

Step 41: Get an alignment!

The alignment is critical to the car's handling and tire wear. After reinstalling the struts, you will lose the previous alignment because there is play in the two strut-to-knuckle bolts. Get an alignment right away or your car may handle funny and wear out its tires quickly.