I have a “Day & Night” BDP (Carrier) furnace that had been working fine. BDP 395AAW036055. The furnace had been providing heat and then I noticed one day that the blower motor kept running and it was only blowing cold air. Now if I turn it on the blower just keeps running and does not respond to the thermostat. The control board had smoke marks so I replaced the board. I also replaced a limit switch (with a manual reset) which had an open loop. However, the blower still keeps running and won’t stop. Also, I am unable to get any voltage at the control board terminals that head out to the thermostat. The transformer seems ok. I get 117 on on one side and 27v on the low side. Am I missing another switch? Can a limit switch or pressure switch deny voltage the terminal? Any help would be appreciated. Thanks! [img]https://cimg1.ibsrv.net/gimg/www.doityourselft.com-vbulletin/828x1792/d6d2b09c_af06_4147_9e33_10ab4159934f_caa3261ba6df01f6a39d976ca500df46bba74393.png[/img] [i]Furnace pic[/i] [img]https://cimg4.ibsrv.net/gimg/www.doityourselft.com-vbulletin/2000x1504/8986c6e6_f23d_4eff_a987_6fe1f61dded7_f399b8e0947b41d82e568497b58fa585ee5f157c.jpeg[/img] [img]https://cimg6.ibsrv.net/gimg/www.doityourselft.com-vbulletin/2000x1504/cba4c996_7b71_4ec3_a078_3df4faf54323_582cc6bba5f10cfec629dfe4ca1ff7507da4fee2.jpeg[/img]

I'm having a weird issue with my gas furnace and I can't seem to figure out what's going on from the service manual. The best I can figure is that it's a bad board. A few days ago, my Nest thermostat disconnected from Wifi and after I reset it, I couldn't get any heat. When I turn on the heat, nothing happens. No inducer motor, no blower, no gas, no ignition. It's like birds chirping. However, the blower and AC work fine when those are individually selected. I tried jumpering W1 to R on my board and it's the same thing, so it doesn't appear to be the thermostat. The status light on the board is solid red indicating normal operation. I checked the pressure circuit and it's open (24V differential measured across it, and infinite resistance), which is what it's supposed to be according to the startup sequence. I thought it might be a bad inducer motor, but the motor spins freely and it measures continuous with my multimeter. I also tried hooking up my multimeter to the power terminals for the inducer motor on the board. When I call for heat using the jumpered connection, I get 1V across the terminals when they are disconnected from the motor. I also tried hooking up the multimeter in sequence with the inducer motor and got 0 amps when calling for heat. All rollouts have continuity and the control limit switch shows continuity as well. I measured the voltage across the Rh and W1 pins and it shows 28VAC, which is within the range I expect. Anyone have any ideas what this might be? The only thing that makes any sense to me is that it's a bad control board. My guess is that the board failed and that's what caused the thermostat to misbehave. Resetting the thermostat fixed it, but the board is still shot. Is there anything else that might cause a no heat condition without triggering a blinking status light?