The second of my two Carrier heat pump units is not switching from heat to cool. I pulled the flow reverser valve actuation solenoid off to test it. It was not exerting any magnetic force on my screwdriver, but I noticed that the solenoid is getting a 24 volt signal as it should, when it is open circuit, ie, when I pull the plug off of the solenoid. Whatever is powering the solenoid has developed a high impedance. The compressor comes on, so the actuator is being controlled by its 24 volt source. My first thought was maybe pitting on the relay on the control board that powers the solenoid, but it measures only .3 ohms when the relay is closed. Any ideas for what to look for?