[NewKidTX]

I have a Carrier gas furnace that is not getting gas; the rest of the cycle runs properly, including the HSI glowing, but there is no gas to light, so the system shuts down and I get a 21 error code (gas lockdown). I am showing no voltage to the gas valve at the point in the cycle when control board should signal the valve to open. Up until yesterday, the problem was intermittent; the gas valve would operate properly until the middle of the night (we'd wake up to below-thermostat-setting temperatures and find the LED flashing the 21 code). As of yesterday, the problem occurs with every attempt to run the cycle. Here in TX, temps are daytime 50s, night 35.

The control board is a Carrier HK42FZ004 (yes, one of those obsolete ones that is known to have problems with solder joints, etc.). My conclusion is there is a bad wiring connection or a bad solder connection on the board. A repairman wants $470 to fix ($340 for the new board and $130 for labor), but I can find a board online for about $240 and try to install it myself. Here are my questions:

- Is there anything I should check before assuming the board needs to be replaced?
- I've never replaced a circuit board before. Can someone briefly walk me through the procedure before I commit to buying the part and doing the work myself? I need to judge if I am in over my head or if I can work through it (even if there is pain involved--ha). Specifically, any particular knowledge about converting from the HK42FZ004 to the new 325878-751 would be helpful.

Thank you,
NewKidTX

[rcvjmv]

I have a Carrier gas furnace that is not getting gas; the rest of the cycle runs properly, including the HSI glowing, but there is no gas to light, so the system shuts down and I get a 21 error code (gas lockdown). I am showing no voltage to the gas valve at the point in the cycle when control board should signal the valve to open. Up until yesterday, the problem was intermittent; the gas valve would operate properly until the middle of the night (we'd wake up to below-thermostat-setting temperatures and find the LED flashing the 21 code). As of yesterday, the problem occurs with every attempt to run the cycle. Here in TX, temps are daytime 50s, night 35.

The control board is a Carrier HK42FZ004 (yes, one of those obsolete ones that is known to have problems with solder joints, etc.). My conclusion is there is a bad wiring connection or a bad solder connection on the board. A repairman wants $470 to fix ($340 for the new board and $130 for labor), but I can find a board online for about $240 and try to install it myself. Here are my questions:

- Is there anything I should check before assuming the board needs to be replaced?
- I've never replaced a circuit board before. Can someone briefly walk me through the procedure before I commit to buying the part and doing the work myself? I need to judge if I am in over my head or if I can work through it (even if there is pain involved--ha). Specifically, any particular knowledge about converting from the HK42FZ004 to the new 325878-751 would be helpful.

Thank you,
NewKidTX

I just had a one very similar replaced yesterday, If you can read electrical schematics you should be able to change the board yourself without any problem. The new board comes with an adapter harness and many of the connections are just located in a different place on the new board. You just need to step through the wiring one connection at a time.

[NewKidTX]

Thanks for the tip. Here's the result:
I bought a board on Ebay and it got here in two days. The next morning I installed it. The whole installation took about 1.5 hours, which is pretty fast considering I've never done it before and I was scared I'd break something or discharge static electricity. :) The instructions were extremely detailed and easy to follow.

If you try this yourself:
1) Be sure the problem is the circuit board. No power to the gas valve and numerous online reports about solder joint failures on this type of board made me pretty certain this was the problem. Always check less expensive possibilities first.
2) Be sure gas and power are turned OFF.
3) Be sure to ground yourself and your tools.

First, to be EXTRA careful, I used little stickers to label each wire while still connected to the old board (the instructions include diagrams of both the old and new boards; I got the label name off the old board). Then it was a simple process of removing the old board from the chassis, disconnecting wires, attaching the new board to chassis, installing the wire adapter that comes with the new board, and reattaching wires to the new board. The test sequence is easy--just use a small piece of wire to connect TEST to COMM and the furnace goes through its own check. After the check, I reconnected the thermostat wires. Everything started up just fine on a full run-through; I heard the "click" of the gas valve, then gas flowed and was lit by the HSI.

So, I saved $320 off the repairman's quote, but more importantly, learned a heck of a lot about furnaces (so I'm better off next time), considering I knew NOTHING about furnaces before last week (though I do have an engineering and "fix-it" background). The several hours of researching was worth it. And I didn't even have to bother my husband about it while he was at work. :)

Thanks!
Karen