To repair a heated car seat, replace any blown fuses and a faulty switch, adjust a thermister, replace or solder the heating wire, and replace the car's heater element if necessary. This procedure may, however, vary according to the cause of heat.
First, check and replace any blown fuses. Check the plug connecting the main wiring to the seat, and plug it in if it is unplugged. Clean it of any corrosion or dirt as well. Check the switch sides for voltage with a voltmeter, and replace the switch if the voltage is not 12.
If the seat still heats after fixing the plugs and switch, examine the heating element thermister, and adjust it if it has shifted to reset the seat temperature. Check if the heating wire is broken, and replace or solder it as necessary. Wrap the soldered joint with electrical tape. If the problem persists, replace the whole heater element. To replace the element, unbolt the seat, detach all the wires, and remove the seat from the car. Separate the seat back from the seat base, and remove the seat upholstery to access the element. Remove the old element, and install the new one in its place before reassembling the seat, then test the seat.