Turn off the main power supply to the AC unit. Remove the screws on the AC unit's access cover with a screwdriver and pull the cover off the unit.
Locate the heating element, mounted on the inside wall of the AC unit. The element will resemble a bar with disks and wires running along its length.
Turn the multimeter on to the 240-volt setting. Confirm that the power is off by touching the meter's probes to the heating element's incoming electrical terminals (wires) that energize the heating element (one probe to each terminal). Be sure there is no voltage reading on the meter before you proceed. Disconnect the electrical terminal leads from the heating element with a screwdriver.
Set the multimeter to the ohm setting and touch the leads of the meter to opposite ends of the heating element's electrical terminals. Any reading on the multimeter indicates the heating element is sound. A zero reading indicates the element is faulty.
Turn on the power to the AC unit and set the thermostat to "off."
Turn the multimeter to the 240-volt setting. Touch the leads of the meter to the opposite ends of the heating element's terminals. Use extreme caution that your hands do not contact any metal.
Check the reading on the meter. If the meter reads 240 volts, the heating element is receiving the proper current load, indicating the problem is not with the connection to the heat strip.