Tester(3prong plug in ) says open neutral. When I plug something in to it the blower for the fireplace comes on. We never knew there was a blower. Is this a switch& receptacle in which someone took out the proper sw&outlet?? What do I do??

Tell us what you are doing. It sounds like you have a receptacle that does not work and when you plug something into it the blower motor on a fire place insert comes on. If that is correct,someone screw up trying to connect the fire place insert motor wiring.