If you leave your MillMax in your car during a hot summer or cold winter day, it will not damage your kit's components. The cables are robust and the manufacturer rates its sensors as follow:
Accelerometer: -100 to +356 °F
Hammer: -30 to 250°F
Before using your kit, however, it should be allowed to return to room temperature by bringing inside or in your call with the heater or AC on. One challenge may be the wax used to mount the accelerometer. It will become hard when exposed to cold. Scrape out a small amount and roll it into a ball between your fingers to heat it up. It should then be pliable enough to use.