When you plug in your cummins truck the heater does not work you have changed the 40 amp fuse what else could it be?

Are you are talking about the block heater or the heater fan. It looks like a two part question. The block heater plugs into a house outlet. The cord starts at the front bumper area and goes to the block. A element goes into the block to heat the engine coolant. To trouble shoot: Power from the house plug in a lamp. Cord from the bumper to block heater, Inspect for damage & ohm out the wires. If all checks fine the block heater maybe defective. Now for the fuse. Is it blowing out all the time & does it go to the heater fan.