AC Thermostat Power Turns Off By Furnance

[RoyalFlush]

The power light on my thermostat turns off automatically. I have to go to the attic to open the furnance and unplug/plug the wires that connect to the curcuit board then the power light comes on. However a few days or weeks later, the light goes off again. And I have to unplug/plug the wires again to get the power light on. I got an AC guy to have replaced the curcuit board but the problem still exists. The ac guy now suggests to replace the whole furnance. But I feel there should be an easy way to get the existing furnance fixed. Can someone help give some tips on how to root cause the issue?

Thank you in advance.

[HayZee518]

furnace

I don't see how replacing the furnace is going to solve the thermostat power issue. the thermostat is the first thing inline with the 24 volt supply to the rest of the circuit. it is the brains of the furnace.
look at your circuit diagram for a time delay circuit for the supply voltage to the "stat." that would be the only thing that could be at fault.

[RoyalFlush]

I forwarded your answer to the AC guy. Here is what he responded:

Listen carefully. I replaced circuit board which has all new cooling relays and heating relays. Sensors are connected to circuit board. I replaced tstat. Same result gas furnace lights and then shuts down I replaced flames sensor I replaced gas vavlve same result. Troubleshooting paperwork suggesting not properly grounded. Which is in the wiring harness and that could be anywhere. I ask you again did you know about cooling issues before this tenant moved in????

The issue did happen in both cooling and heating modes. My question is if the time delay to the stat usually is not on the main circuit board? The furnace is a 1999 Amana. Thanks.

[HayZee518]

furnace

the guy answered his own question: BAD EQUIPMENT GROUND.
Now if the problem is within the wiring harness, its not something YOU did but the manufacturer. The wiring harness supplied with the unit is bogus. That's why he suggested a new furnace. I mean, how often would a wiring harness be at fault for any three units put into production.
Check the equipment ground. That is the bare wire or green wire or yellow with a green tracer on its jacket and how its applied to the unit and the breaker panel. Its supposed to be connected to the metal chassis of the unit. at the panel, its either at the neutral strip or an auxillary grounding strip that's isolated from the service neutral if the connection is a sub-panel.
voltage spikes go to earth ground. voltage spikes can destroy a control board.