If your microwave oven is over 15 years old, it should be checked for output efficiency. For 600- to 1000-watt microwave ovens, place an 8-ounce cup of water in the oven and operate the unit on high for three minutes. The water should reach a rolling boil. If not, take the microwave to a service shop for inspection.
Older microwaves should also be tested for radiation leakage by a professional appliance repair technician. In addition, the pro can check other aspects of operation to determine if the microwave is safe and if it should be repaired or replaced.
Never attempt to repair an ailing microwave oven yourself. Besides the inherent dangers (the unit's capacitor holds up to 4,000 volts of electricity), unauthorized repairs by anyone other than an authorized service tech almost always void the manufacturer's warranty.
Poor heating in your microwave can result from an overworked electrical circuit. Operating a microwave on a circuit that is serving other appliances not only diminishes the effectiveness of the microwave oven, but could ultimately result in an electrical fire. If lights dim when the microwave is used, an electrician should be consulted. Whenever possible, a separate electrical circuit should be provided for the microwave oven.
Always wipe up spills promptly after use. Keep the interior of the oven and the area surrounding the door clean, using a damp sponge to catch spills and splatters as they occur.
Food particles left over long periods of time eventually turn to carbon and cause arcing (electrical sparking), which in turn can etch interior surfaces and could even compromise the seal around the door.
To help keep the microwave's interiors clean, always adequately vent airtight packaging, plastic bags, plastic-wrapped bowls, eggs, and all fruits and vegetables with skins (like potatoes) by puncturing them or cutting vent slits. Doing so avoids bursting from steam buildups that creates messy food splatters.
Never use a microwave oven for anything other than what it was designed for -- cooking and heating. Using it to dry clothing, papers, or (gasp) pets yields devastating results.
Never run an empty microwave, and always be sure there is plenty of ventilation surrounding the unit to avoid overheating which could cause serious damage.
Appliance repair pros say the most common repair problem they find is a simple microwave fuse that gets metal fatigue after three or four years' use. At that point, even a minor power surge can cause the fuse to burn out. If your microwave quits, don't panic. It may just be an interior fuse that needs replacing by a pro.