[DrBill]

Help Replacing Carrier Circuit Board HK42FZ004

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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
DrBill

[KISS]

Online for $130

HK42FZ004, great deals on Business Industrial on eBay!


In terms of replacing the board:

Observe static precautions. Basically touch the furnace first before touching the board. Don't rub yourself on carpet.

Turn power off.

Mark all wires so you know where they go.

If there are any DIP switch settings, transfew the to the new board.

And above all, MOve the model plug from the old board to the new one. It consists of a couple of resistors on a 4 pin header, otherwise the board inherits the personality of the unit it was last used in unless the board was new.

[KC13]

The replacement kit 325878-751 comes with detailed instructions for installation. Based on the knowledge you have demonstrated so far, it should be relatively simple for you. Don't be too proud to label the wires before removal if it increases your comfort level with this. I have replaced many of these, and always get a kick from the homeowner's expression when I start pulling wires off.

[PDPDPD]

Please make sure gas valve solenoid is good you can externally apply the voltage as per rating on solenoid or measure voltage on board