Microwave ovens have become ubiquitous in most home and restaurant kitchens, and are convenient for preparing frozen foods, defrosting meats, warming up sauces, hot beverages, soup, and making popcorn.
But how long before microwave ovens need to be replaced?
Microwave ovens use powerful electromagnetic waves to heat up food. The source of a microwave oven’s power is a magnetron tube that uses magnetic and electrical fields to create oscillating waves.
Over time, the microwave oven’s magnetron tube weakens and becomes less effective, so the microwave oven’s power slowly diminishes.
Some claim a typical magnetron tube has an average life of 2,000 hours of operation, and suggest that if you use a microwave oven for a total of 15 minutes every day, it would take 22 years for it to reach the 2,000-hours-of-operation point.
According to Mr. Appliance, the life span of a microwave oven is 5-10 years with an average of 8 years.
Ela Schwartz with Home Sweet Solutions claims there are three major signs to watch for that may indicate it’s time to replace your microwave oven:
1. Your microwave makes loud noises while cooking. 
This could be caused by something as simple as a wayward piece of food stuck in the turntable or misaligned fan blades. It could also be caused by a defective power diode, magnetron or high voltage capacitor. Check the turntable — if you don’t see anything, it’s time to replace your microwave.
2. You punch in the cook time, press Start and get nothing but a loud buzzing.
This could mean a defective power diode, high voltage capacitor or magnetron. Replace your microwave oven immediately.
3. Your microwave works, but cooking times take longer.
Try the two-minute test: an 8-ounce mug of water should be steaming hot after two minutes. If it isn’t, the magnetron is starting to fail, and it’s time to replace your microwave.
Schwartz also mentions three problems that may be fixable:
1. There’s no light when the microwave starts cooking. You may just need a new light bulb. Since this involves opening the microwave’s cabinet, call a repair shop and get a price quote.
2. Your microwave oven door has problems. The door to your microwave oven is a vital safety component that’s carefully designed to contain radiation. The door interlock switches a signal that the door is shut, which means it’s safe for the unit to start heating.
If your microwave runs when the door is open, or stops working after you close the door, check the door hooks and door release button to see if they’re broken, as you might have damaged an interlock switch by slamming the door too hard.
Keep in mind that the door interlock switches work together. If one is damaged, you’ll probably have to replace all of them.
3. The touch pad isn’t working properly. If nothing happens when you hit Start, or certain keys don’t function, the touch pad may need to be removed and either cleaned or replaced. A new touch pad could cost practically as much as replacing your microwave oven.