A built-in dishwasher can make cleaning the kitchen after cooking and eating a much quicker task. Many new dishwashers are far more efficient than older models, so replacing an old dishwasher can result in cost savings on both the amount of water used and the heating of the water. You can remove the old dishwasher and install a replacement built-in dishwasher in about an hour with some basic hand tools and a new supply line.
Turn off the electrical circuit to the dishwasher at the circuit breaker or electrical panel.
Turn off the hot water supply valve under the sink. Loosen and remove the hot water supply line from the valve with an open-end wrench. Loosen the clamp on the garbage disposal with a screwdriver and remove the drain line.
Remove the bottom panel of the dishwasher by loosening the screws on the front of the panel. Reach under the dishwasher, and open the electrical junction box. Disconnect the lead, common and ground wires from the dishwasher.
Open the front of the dishwasher, and remove the screws that secure the dishwasher to the underside of the cabinet. If there are any mounts behind the bottom compartment of the dishwasher, remove the screws from those mounts as well.
Slide the dishwasher out of the cabinet while easing the drain line and supply lines through the holes in the side of the cabinet. Once the hoses are free, pull the dishwasher completely out of the cabinet opening, and move to the side.
Remove all wrapping from the new dishwasher, and slide it to the opening in the cabinet. Remove the lower front panel, and look either underneath or behind the dishwasher for the supply line connection. Apply two to three wraps of Teflon tape around the connection, then tighten a new water supply line onto the connector with an open-end wrench.
Feed the supply line and the drain line through the hole(s) in the side of the cabinet, into the under-sink cabinet. Push the dishwasher about halfway into the cabinet, and then reach under the sink and pull out the two lines to ensure that they're not being pinched by the movement of the dishwasher. Push the dishwasher the remaining way into the cabinet, then pull the supply and feed lines the rest of the way into the under-sink cabinet.
Apply two to three rounds of Teflon tape to the hot water connection, and connect the supply line. Tighten the supply line connector with an open-end wrench.
Connect the drain line from the dishwasher to the drain receptacle on the side of the disposal. Tighten the clamp with a screwdriver to prevent leaks.
Reach underneath the new dishwasher and find the electrical wires. Connect the wires to the wiring in the junction box (black to black, white to white, copper to green). Attach new wire nuts to each connection, then tuck the wires into the junction box and replace the junction box cover. Tighten the screw to secure the cover.
Reposition the dishwasher so that the door is flush and even with the cabinet. Attach the dishwasher mounting brackets to the cabinet with a screw through each bracket into the cabinet.
Turn on the hot water supply line, and look and feel for leaks. If you don't find any, turn on the electrical circuit at the breaker panel or fuse box, and run the dishwasher for a test cycle. Check to be sure that there are no leaks on the supply or drain lines, both under the kitchen sink and beneath the dishwasher.
Replace the bottom panel of the dishwasher, and secure it in place by tightening the screws.