I have a 2006 f-250 SD with 6.0 and block heater does not appear to do anything. I plug in but warm starts are not happening.

How long do they need to be plugged in? I have left it plugged for 4 hours before and nothing.

Am I doing something wrong? One end into the plug, the other end into live outlet.

 

Guess I don't quite understand your defination of 'warm'. I understand it's main purpose is essentially to get the coolent above 50 degrees or so. Even when I've left mine plugged in overnight, the needle on the temp guage will point to the bottom mark, but drop as soon as I start the truck as now that tiny area of semi-warm coolent is now diluted by all the rest of the cold coolent in the rest of the system. Too be honest, I haven't seen much of a difference in starting at 20 degrees with our without the block heater plugged in for a few hours ... maybe at 0 or below, I would sing a different tune about whether or not it help.