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House and furnace are 5 years old. Furnace is an Allied A95UH2V090C16S-07 and is interlocked to a Lifebreath HRV. Functioned fine up until a couple days ago.

My wife calls me and tells me that the furnace isn't working. I get home from work and start looking at it. When cycled it will run the inducer, then the ignitor glows for about 20 seconds, then click, nothing, ignitor shuts off and the LED status light starts blinking. Status LED is blinking 3 times which is the code for low-fire pressure, rollout, or aux limit switch open so I started looking at vents for blockages but there weren't any. Cleaned out the condensate drain and made sure it was flowing freely. Still no dice.

This morning I decide to disconnect the HRV to help isolate the problem and BAM, furnace ignites properly and runs fine. I checked the relay on the HRV with a multimeter and it's working fine, both the NO and NC contacts open and close correctly.

Move back to the furnace and check the G terminal when a command for heat is active, G is not active. This is fine because the furnace activates the fan automatically when there is a call for heat. However, G is active when the HRV is interlocked any time the HRV is circulating air.

To test I jumpered R to G on the furnace terminals and presto, furnace will no longer ignite. The ignitor glows but then after 20 seconds it trips out and flashes the LED 3 times again. Removed the jumper, connected the G terminal on thermostat directly to the furnace G terminal, selected fan mode "On" on the thermostat, same problem as with the jumper from R to G. Turn the thermostat fan mode to "auto" and then the furnace ignites properly again because the G terminal is not active.

Do I have a bad control board or am I missing something? A command for fan on G should not interfere with the furnace ignition?

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