No dip the probe into the coolant and ground the negative lead to the body or negative battery cable .
Also get a test strip kit and see if the coolant still has it's protective qualities .
The coolant takes a charge and nearly instantly starts the process of loosing it's ability to be a protectant against PH changes that become corrosive , the older the coolant the faster it will be a determent .
Awareness of this is lost on most people until you have a continuing problem .
Mine was caused by a bad Optima battery that in turn caused a starter to fail internally then reared its ugly head as a radiator leak .
I had experienced this in the past on other vehicles , and then had an Ah-Ha moment and discerned that I had a problem .
Finding cause and effect was another story , I had a bunch of great help from other JKO members that did many hours of testing and many hours of labor to get to the root problem .
Those may or may not chime in on this (don't know) , but without them I'd still be changing out radiators .