When a microwave quits heating, start with the kind of unit and the cost of a comparable replacement. For a countertop model, a new unit is often cheaper than internal work. For a built-in or over-the-range model, installation can change the math, but the high-voltage safety issue does not change.
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 parts.
Before assuming the magnetron, a technician checks the cheaper possibilities:
Door interlock switches. A stuck or failed switch can cut power to the magnetron while the fan and turntable continue to work.
Thermal cutout or thermoprotector. An overheat event can open a thermal fuse.
High-voltage diode and capacitor. These parts work in the circuit that powers the magnetron. A failed diode can be less expensive than a magnetron, but reaching it is not safe without proper training.
Control board. A failed board can prevent the cooking cycle from working even when the magnetron is intact. On a budget 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 control board.
The sequence also explains the decision. A cheap component may be worth quoting, while a magnetron on a low-cost countertop unit can exceed the price of a new microwave.
What you should not do yourself
The high-voltage capacitor can store a lethal charge even when unplugged. A built-in bleeder resistor does not prove that the terminals are safe.
Safe homeowner checks are limited to the outlet, the latch, the power setting, and a dirty waveguide cover. Do not open the cabinet or touch the magnetron, diode, capacitor, or transformer.
The cost decision, laid out simply
These are rough typical Bay Area replacement ranges, not a quote for internal work:
Countertop microwaves. A unit under about $200 new is usually better replaced. If a paid internal repair approaches half the replacement price, replacement wins.
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 in the comparison.
Built-in and microwave-convection units. Cabinet fit and installation can make a new unit more expensive than the sticker price. That does not make case-off repair safe for a homeowner.
If the microwave arcs or sparks, stop using it. A damaged waveguide cover can be visible, but smoke or a burning smell means unplug the unit immediately.
When to choose replacement
Microwave repair is not a service ADRIUM offers, including countertop, over-the-range, and built-in units. For other appliance or home-maintenance questions, use the Home Service Pros information resource.