The short answer
On a Thermador microwave this is usually an interlock switch not making rather than the magnetron, diode, capacitor or transformer. The first checks below cost nothing and need no panel off; the one at the bottom of the list is a service call.
What you actually observe
- The turntable turns, the light is on and the food stays cold — Everything except the heating is working, which points squarely at the high-voltage side.
- It heats for a few seconds and then stops — A thermal cut-out, usually from a blocked vent.
- NEAR MISS: it heats but sparks inside — Stop now. Unplug it and do not use it until it has been looked at.
Causes, most likely first on this build
- An interlock switch not making — The oven will run its light and turntable on one switch and withhold the high voltage on another, so a partly closed door gives exactly this. First on Thermador because this platform senses what a simpler one switches, and its doors are heavy enough to move the geometry.
- A latched control or a demo mode — Some units have a showroom mode that runs the display and the turntable and never fires the magnetron. Second here: a board decides the behaviour rather than a switch, so a control fault reads as a machine declining to act.
- Blocked vents cutting out on temperature — The magnetron is air-cooled. Blocked vents overheat it, a cut-out opens, and the oven runs on without heating. Third, once the two above are clean: a built-in cavity vents through a path the cabinet defines, and that path blocks before anything else does.
- The magnetron, diode, capacitor or transformer — The high-voltage circuit has failed, and that is what is left once supply, venting and the door are clean. Bottom of the list on purpose: heat is made by one circuit and protected by another, and either can stop it.
What to check first, second and third
- Close the door firmly and check it sits square with nothing on the seal — Never tape, bend or defeat an interlock to test it.
- Unplug for a few minutes, plug back in and check for a demo indicator — Free, and it explains an oven that has never heated since it arrived.
- Check the vent grilles on the cabinet are clear and that it has clearance — A cut-out that resets when cool gives an intermittent fault.
The checks you can do yourself
- Close the door firmly and check the seal face and the latch hooks are clean and undamaged — If the door or latch is damaged, stop using the oven. There is no safe home test.
About 10 min
- Unplug for a few minutes, plug back in, then check the panel for a demo or showroom indicator — A new oven that has never heated is very often this.
About 10 min
- Clear the vent grilles and give the cabinet the clearance the maker asks for, then test cold — A built-in unit with a blocked cavity vent does this reliably.
Tools: a vacuum, a soft brushAbout 20 min
- STOP HERE: the high-voltage capacitor holds a lethal charge even unplugged — There is no owner check past this point. Call, or replace the oven.
Read this before you start
Safety
Unplug the microwave before removing any panel. The high-voltage capacitor inside holds a lethal charge after the power is off and it does not discharge on its own. Never defeat, tape or bend a door interlock to make the oven run with the door open, and never run it empty. If the door, the latch, the seal or the window is damaged, stop using it: that is the barrier, and there is no safe way to test it at home.
When to call a technician
Call as soon as the oven runs normally and simply will not heat: that is the high-voltage circuit and it is not an owner repair at any skill level. Stop using it immediately for sparking, a burning smell, a damaged door or seal, or a latch that does not hold. On many units a repair costs more than the oven, so ask for the estimate first.
Nearest pages of this class
What is deliberately not on this page
This page covers the Thermador microwave as a class, not one family or model line. Where a family-specific page exists it is linked below and it is the better answer. Not here: magnetron part numbers, test points and any high-voltage procedure, which are deliberately omitted.
Lead Appliance Repair Technician · 12 years experience
Experienced technician with 12 years specializing in washing machine and dryer repairs across all major brands.
