GE over the range microwave, model JNM3151DF1BB, 15 months old. Exhaust fan will not turn off, must unplug the unit after each use. Still heats food normally. I’ve replaced both thermostats on the bottom panel of the unit but the fan still runs continuously. I’ve tested all 3 door switches and they check out ok. I’m guessing the control board is bad, in which case I will probably replace the whole microwave rather than gamble on such an expensive part. But before I do that, is there anything else I should be testing? Thanks!

Thanks, that's what I was afraid of. I'll probably just go ahead and buy a new microwave...this time with an extended warranty.