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BillyHvac, Journeyman HVAC Tech
Category: HVAC
Satisfied Customers: 22002
Experience:  Endorsed for unlimited heating, cooling, oil burners, boilers, refrigeration, hydronics
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CNT03078 2-stage Control Board - No 24 VAC at Gas Valve LO

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CNT03078 2-stage Control Board - No 24 VAC at Gas Valve LO input
24 VAC good.
TCO switches good.
Flame Roll-out switches good.
PS First Stage good.
Ignitor energizes and heats up brightly.
No 24 VAC from Pin 7 MVL.
Thus, solenoid does not kick on.

If I jumper 24 VAC from transformer to Gas Valve LO input after ignitor energizes, then solenoid energizes normally, gas flows, and I have main burner ignition.

Where does the 24 VAC to MVL Pin 7 come from? Does it come from the microprocessor directly, or does the microprocessor energize one of the relays which in turn provides the 24 VAC to MVL Pin 7?



Do you have a thermal cut code blinking when this occurs?

Or do you end up with failed ignition attempt?


Customer: replied 2 years ago.

I'm not sure what a thermal cut code is.


I get an external lock-out after re-tries are exceeded (2 flashes). But LED goes back to Normal Call for Heat (flashing 2 times per second) once I remove power then turn back on which resets the control card.


When I jumpered directly to the solenoid from the transfer, and main burner ignited, I got a LED code for Flame Sensed When No Flame Should be Present (5 flashes).

Thank you.

The relay fo low is bad in the board itself.

by bypassing power to coil proves valve ok but it cannot circumvent the internal fault.

the board"thinks" it sent 24v but flame failed


Customer: replied 2 years ago.

Can you point to which relay is the culprit?

Can I obtain and replace the relay without too much trouble? I have the ability to properly solder. (Used to teach it...)


I would rather pay for and replace a relay, than spend $300 on a new board.


Thanks Billy.

I would love to be able to do that...

However I am restricted from instructing (or doing myself) any repair/alteration to a circuit board that has flame sense incorporated into the circuitry.

I am not even able to obtain prints of the board as to the layout or design.

I am very sorry but it becomes a liability issue if the circuit board is serviced other than by manufacturer.

Customer: replied 2 years ago.

Well...I do understand. Makes sense to me from a liability perspective.


However, I'm sure you understand my position as well. :o) Replacing a relay is not rocket science...


I've got a detailed verbal circuit description. It just doesn't tell me where the 24 VAC comes from. It says only, "The control then switches 24 volts to its "MVL" and "MV COM." I figured it had to come from a relay rather than the IC itself. And the relay is switched by the IC.


Thanks for your help Billy. I'm still in the same boat. Unless I can figure out which relay the 24 VAC comes from, I will have to replace the entire board. Crazy.


I understand totally.

Especially with the technology of all the boards now.

Unfortunately it is proprietary and I cannot even source the parts on the board.

I wish you the best.


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