The answer first
A microwave that runs its full time, lights up and turns the plate but leaves the food cold has lost its magnetron output, and there is no owner repair for that — but there are three things worth checking before accepting it. On a Criterion a shared or undersized supply comes first, because these units are frequently owner-installed in a closet, a garage or a basement corner, so what feeds the machine is as suspect as the machine. Confirming the fault, checking the door and ruling out the setting are owner work; nothing inside the cabinet is, at any point.
What you observe
- Food warms slightly on a long cycle — Partial output, which usually means the magnetron is on its way out.
- A loud buzz or hum while it runs, and still no heat — The high-voltage side is energised but not producing output.
- The oven heats on one power level but not another — A control or relay issue in how the output is switched.
- NEAR MISS — Food is hot outside and cold inside — That is how microwaves cook; stirring and standing time, not a fault.
Why it happens, in order
- A shared or undersized supply — Most likely here: these units are frequently owner-installed in a closet, a garage or a basement corner, so what feeds the machine is as suspect as the machine. A microwave draws heavily; on an overloaded circuit or a long extension lead the voltage sags and output falls without the oven appearing faulty.
- A power level, defrost or demonstration setting — Second on the list: A microwave left on a low power level or in a shop demonstration mode runs completely normally and delivers almost nothing to the food.
- A door interlock that has half-failed — Third on the list: The interlocks decide whether the oven may generate at all; when one fails, many models run every function except the magnetron.
- A high-voltage diode, capacitor or transformer fault — Fourth on the list, and the least likely: These feed the magnetron and they fail in ways that produce the same cold result, sometimes with a buzz or a blown fuse. The drivetrain is simple, which makes it durable — it is a real cause but rarely the first one.
Check in this order, and why
- Plug it straight into a wall outlet on its own and retest — An oven that works better on a different outlet is telling you about the circuit rather than itself. It goes first because it is the cheapest thing to rule out and it decides which half of this list you are in.
- Set full power and a short time, and heat a cup of water — Water that stays cold after a couple of minutes at full power is a real fault; water that heats means the setting was the problem.
- Look at whether the door closes squarely and latches firmly — A sagging door or a damaged latch is visible, and it matters more on this appliance than on any other.
Do these, in this order
- Remove any extension lead and test in a wall outlet with nothing else on the circuit — This is the only supply-side check worth doing; everything else on this appliance is behind the cabinet.
About 10 min
- Reset to full power and clear any demonstration or child-lock mode, then test with a cup of water — Use a microwave-safe cup half full, never an empty oven — running empty damages the magnetron.
Tools: cup of waterAbout 10 min
- Stop here if the next move needs a meter on live terminals — That is where an owner check ends and a service call begins.
What to do before a panel comes off
Safety
Unplug the appliance or switch off its breaker before removing any panel. A capacitor can hold a charge after the power is off. Nothing here asks you to take the outer cabinet off, and a door interlock is never taped or defeated.
Where an owner fix ends
Call as soon as a cup of water fails to heat at full power on a sound supply. Everything beyond that point is inside a cabinet containing a capacitor that can hold a lethal charge, and it is not an owner repair under any circumstances.
Other faults on this machine
What is deliberately not here
This page covers Criterion microwaves as a class, not one family or model line. Nothing more specific exists to send you to at the moment; a model-level page, once written, is linked here and is the better answer. Deliberately not here: wiring colours, resistance values, torque figures and part numbers, which differ between families.
Lead Appliance Repair Technician · 12 years experience
Experienced technician with 12 years specializing in washing machine and dryer repairs across all major brands.
