GM 6.5 Diesel Block Heater
Updated December 18, 2016
All the trucks have 600 watt block heaters that are inserted in one of the engine freeze plugs in the water jacket (see below). There should be a 3 prong plug sticking out at front bumper level on the drivers side. It's not easy to see. Plug it into an extension cord if it's going to be colder than 25 degrees f and your truck is parked outside.
Many of the heaters have never been used. The plugs can be dirty and corroded. Clean them off with a wire brush or replace the end.
How do you know if it works. You should hear a gurgling or hissing sound (like a small coffee pot). Some say you can reach up and feel the heat after its been on for a while. Next put an ohm meter across the plug and see if you get something. If your meter shows that it's open either a wire is broken or the heater element is no good. Next I'd get a current tester (A/C clamp meter) and see if you are getting a good draw on your AC. You should it draw around 5 amps. If you're not getting any current draw the heater may be bad.
Below block heater shown in the block