When a microwave quits heating, compare the cost of a safe replacement with the cost of internal work. On a countertop model, a new unit often wins. On a built-in or over-the-range model, installation can change the math, but the high-voltage safety issue remains.
Why it stopped heating, most likely first
The magnetron makes the microwave energy. It is a common reason a unit runs, spins the plate, lights up, and produces no heat. It is also one of the more expensive internal components.
Before assuming the magnetron, a technician checks:
Door interlock switches. A failed switch can cut power to the magnetron while the fan and turntable continue working.
Thermal cutout or thermoprotector. An overheat event can open a thermal fuse.
High-voltage diode and capacitor. These parts work in the magnetron circuit. A diode can be less expensive than a magnetron, but reaching it safely requires training and proper tools.
Control board. A failed board can stop the cooking cycle even when the magnetron is intact. On a low-cost unit, the board can push the total beyond replacement value.
How a technician narrows it down
A sound diagnosis checks the door system first, then the high-voltage circuit and wiring. That avoids buying a magnetron when the actual problem is a switch, fuse, or board.
What you should not do yourself
The high-voltage capacitor can store a lethal charge even when the microwave is unplugged. A built-in bleeder resistor is not proof that the terminals are safe.
Safe checks are limited to the outlet, power setting, latch, and a dirty waveguide cover. Do not open the cabinet or touch the magnetron, diode, capacitor, or transformer.
The decision, laid out
These are rough typical Bay Area replacement ranges, not a quote for internal work:
Countertop microwaves. Under about $200 new, replacement usually wins. If internal work approaches half the replacement price, replace the unit.
Over-the-range microwaves. A new unit may cost about $600 to $1,500 with installation in the Bay Area. Include removal, mounting, venting, and electrical work.
Built-in and microwave-convection units. Cabinet fit and installation can make a new unit more expensive than its sticker price. Case-off repair is still not a homeowner project.
If the microwave sparks, smokes, or smells like burning plastic, stop using it and unplug it.
When to choose replacement
Microwave repair is not a service ADRIUM offers, including countertop, over-the-range, and built-in units. For more home-maintenance guidance, see Home Service Pros.