Frigidaire FG6RK 096C-16B Indoor Fan switches from high to low speed in cooling mode


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I have a Frigidaire furnace and lately I noticed low airflow coming out of my grilles and my house was taking a long time to cool down. First I suspected a broken supply duct, but that was not the case. Then I thought to check my furnace control board. I check the voltage outputs to the indoor fan. I have a 4 speed fan, but my board only uses the cooling and heating speed outputs. The other 2 fan speed wires are "parked" on dummy terminals which the board does not use. When my thermostat calls for cooling, the indoor fan starts on the cooling speed and then after about 2-3 minutes the fan speed relay clicks (I can hear it) and the fan speed slows down. The output on the board actually switches from cooling (high) speed to heating (medium/low) speed and then my system runs in the heating speed continuously until the thermostat is satisfied.

I have a single-stage thermostat (Nest), so there are no changes to the low voltage thermostat outputs that could be triggering the speed change. The Nest just calls for cooling and fan on. I've checked for 24 VAC at the Y (cooling) and G (fan) terminals and I'm good there, even when the speed changes.

I replaced the control board thinking that was the problem, but the new board does the same thing. Any HVAC techs have any thoughts?
 
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