To clear some misunderstanding here is an excerpt from Wayne Shirley's (Heat Pump Specialist for RSES) book:
"With time-temperature controls the outdoor coil sensor switch is usually the first to go. These switches are simple bi-metal devices that clamp to the coil tubing. They suffer through wide temperature swings as well as lots of moisture, so it’s understandable that they could fail. And some of them, due to design, simply fall apart...I quite often see the “shell” still attached, while the guts of the device are lying in the bottom of the condenser....But with these sensors it’s usually a fairly simple procedure to check whether or not the switch is closed, either by measuring for continuity or voltage drop across the leads. And if it’s covered in frost or ice, it should be closed. If you need or want further evidence to eliminate the board, jumper across the sensor connections, in effect “closing” the switch, and then short across the defrost board test pins. If the control initiates a defrost cycle, the board should be good. Of course, if you find the switch apparently closed and jumpering the test pins fails to initiate a normal defrost cycle, you have to assume the board is faulty. Before doing that, switch the reversing valve with the control wiring or stat to eliminate a bad reversing valve, or valve solenoid."