What can go wrong
The most common problems occur in the microwave generating portion of the
system, though the controller can be blown by a lightning strike or other power
surge. Bad interlock switches probably account for the majority of microwave
oven problems. Also, since the touchpad is exposed, there is a chance that it
can get wet or damaged. If wet, a week or so of non-use may cure keys that
don't work. If damaged, it will probably need to be replaced - this is
straightforward if the part can be obtained, usually direct from the
manufacturer. Unfortunately, it is an expensive part ($20-50 typical).
The interlock switches, being electromechanical can fail to complete the
primary circuit on an oven which appears to operate normally with no blown
fuses but no heat as well. Faulty interlocks or a misaligned door may result
in the fuse blowing as described above due to the incorrect sequencing of the
door interlock switches. Failed interlocks are considered to be the most
common problems with microwave ovens, perhaps as high as 75% of all failures
No adjustments should ever be required for a microwave oven and there are no
screws to turn so don't look for any!
General system problems
The following problems are likely power or controller related and not in
the microwave generator unless due to a blown fuse or bad/intermittent
- Totally dead oven.
- No response to any buttons on touchpad
- Oven runs when door is still open.
- Oven starts on its own as soon as door is closed.
- Oven works but display is blank.
- Whacked out controller or incorrect operation.
- Erratic behavior.
- Some keys on the touchpad do not function or perform the wrong action.
- Microwave oven does not respond to START button.
First, unplug the microwave oven for a couple of minutes. Sometimes, the
microcontroller will get into a whacko mode for some unknown reason - perhaps
a power surge - and simply needs to be reset. The problem may never reoccur.
Note: when working on controller related problems, unplug the connection
to the microwave generator (HV transformer primary) from the power relay
or triac - it is often a separate connector. This will prevent any possible
accidental generation of microwave energy as well as eliminating the high
voltage (but not the AC line) shock hazard during servicing.
If this does not help, there is likely a problem with the controller circuitry
or its power and you will have to get inside the oven.
Some cockroaches (or other lower life forms) may have taken up residence on
the controller circuit board. It is warm, cozy, safe, and from their point of
view makes an ideal habitat. If you got the microwave oven from a flea market,
garage sale, the curb, a relative, or friend, or if your kitchen isn't the
cleanest in the world, such visitors are quite possible. Creatures with six
or more legs (well, some two legged varieties as well) are not known for their
skills in the areas of housekeeping and personal hygiene.
Clean the circuit board and connectors thoroughly with water and then isopropyl
alcohol. Dry completely. Inspect the circuit traces for corrosion or other
damage. If there are any actual breaks, these will have be be jumpered with
fine wire and then soldered. Hopefully, no electronic components were affected
though there is always a slight possibility of other problems.
Totally dead oven
First, check power to the outlet using a lamp or radio you know works. The
fuse or circuit breaker at your service panel may have blown/tripped due to
an overload or fault in the microwave oven or some other appliance. You may
just have too many appliances plugged into this circuit - microwave ovens are
high current appliances and should be on a dedicated circuit if possible. If
you attempt to run a heating appliance like a toaster or fryer at the same
time, you *will* blow the fuse or trip the circuit breaker. A refrigerator
should never be plugged into the same circuit for this reason as well - you
really don't want it to be without power because of your popcorn!
If you find the fuse blown or circuit breaker tripped, unplug everything from
the circuit to which the microwave is connected (keep in mind that other
outlets may be fed from the same circuit). Replace the fuse or reset the
circuit breaker. If the same thing happens again, you have a problem with
the outlet or other wiring on the same branch circuit. If plugging in the
microwave causes the fuse to blow or circuit breaker to trip immediately,
there is a short circuit in the power cord or elsewhere.
The microwave oven may be powered from a GFCI outlet or downstream of one and
the GFCI may have tripped. (Removing a broken oven lamp has been known to
happen.) The GFCI outlet may not be in an obvious location but first check
the countertop outlets. The tripped GFCI could be in the garage or almost
anywhere else! Pushing the RESET button may be all that's needed.
Next, try to set the clock. With some ovens the screen will be totally blank
following a power outage - there may be nothing wrong with it. Furthermore,
some ovens will not allow you perform any cooking related actions until the
clock is set to a valid time.
Assuming these are not your problems, a fuse has probably blown although
a dead controller is a possibility.
If the main fuse is upstream of the controller, then any short circuit
in the microwave generator will also disable the controller and display.
If this is the case, then putting in a new fuse will enable the
touchpad/display to function but may blow again as soon as a cook cycle
is initiated if there is an actual fault in the microwave circuits.
Therefore, try a new fuse. If this blows immediately, there may be a
short very near the line cord, in the controller, or a defective triac
(if your oven uses a triac). Or, even a shorted oven lamp - remove and
inspect the light bulb and socket.
If it does not blow, initiate a cook cycle (with a cup of water inside). If
the oven now works, the fuse may simply have been tired of living. This is
If the fuse still blows immediately, confirm that the controller is
operational by unplugging the microwave generator, power relay, and/or
triac from the controller. If a new fuse does not now blow when a cook
cycle is initiated - and it appears to operate normally - then one of
the components in the microwave generator is defective (shorted).
Some models have a thermal fuse as well and this may have failed for no
reason or a cooling fan may not be working and the oven overheated (in
which case it probably would have died while you were cooking something
for an important guest - assuming you would use a microwave oven for such
Other possible causes: bad controller power supply or bad controller chip.
Totally dead oven after repair
On some microwave ovens, there is at least one cabinet screw that is slightly
longer than all the others. This engages a safety interlock which prevents
the oven from receiving power if the correct screw is missing or in the wrong
hole. Check the length of all the screws and locate the interlock switch
behind one of the screw holes. I don't know how common this practice is
but have heard of it on some Sharp models.
The most common way that the controller circuitry can be harmed is by a power
surge such as from a lightning strike. Hopefully, only components on the
primary side of the power transformer will be affected.
- Check the primary of the power transformer - if it is open, there may be a
fuse/thermal fuse underits outer insulation. If not, the transformer will
need to be replaced. There is a good chance that the surge didn't propagate
beyond the transformer and thus the rest of the controlled should be
- In some cases, circuit board traces may have been vaporized (but repair may
still be possible by simply jumpering across the crater). Some of these
thin traces may be there specifically to act as fuses - and there may even
be spares to use for just this situation!
- Assuming that the main fuse and power transformer primary checks out, then
check the power supply for the controller next.
- As always, also check for bad solder connections.
If the controller power supply is working and there is still no sign of life
(dead display and no response to buttons) the microcontroller chip or some
other part may be bad. It could be a simple part like a capacitor or diode,
but they would all need to be tested. At this point, a schematic of the
controller board will be needed - often impossible to get - and replacement
controller or even just the main chip may be nearly as expensive as a complete