To be honest in all my years I have never actually seen one in use personally. That may be because they showed up in fairly common use after I got away from working as a mechanic or teaching mechanics. I'm not even sure that the collection of automotive text books that I have even shows how to use one as the last one of those is from about 1988.
They work good. Have used that many time back when I ran my shop. The stuff I used is a blue liquid that will change to green if CO is present.
Heck I always look into the radiator for bubbles coming up. But then when I blow a head gasket there is really no point of looking because you already know what I did
The Jalopy Journal
