The short answer
A B machine that runs, lights and turns but does not heat has a specific short list, and two of the items on it are free to check. On a countertop machine the supply and the setting are both easy to prove, which is why they come before anything inside the case.
What you observe on these models
- The turntable turns and the light works but the food stays cold — The machine is running: the fault is in the heating path, not in the power path.
- It heats normally on some settings and not on others — That behaviour belongs to the control path rather than to the magnetron.
- Food is hot at the edges and cold in the middle — Load shape and rotation before anything electrical.
- It heats for the first minute and then stops heating — A protection device operating rather than an outright failure.
- NEAR MISS — nothing at all happens when start is pressed — That is a no-start fault and belongs on a different page.
Causes, most likely first on the B family
- The door interlock chain not fully making — First on this platform: microwaves use more than one switch in the door, and on many designs the machine will light, turn and count down with one of them unmade while the magnetron is never enabled.
- Power level, cook time and the mass of what is in the cavity — Second: a machine set to a low power level cycles its output on and off, which feels like weak heating rather than none.
- The magnetron and the high-voltage side that feeds it — Third: this is the commonest hardware failure behind a machine that runs and does not heat, and it sits below the free checks only because it is the one that costs money to prove.
- Control board, relay or the touch panel behind it — Fourth: a machine that heats on one setting and not another, or that has stopped responding to one part of the panel, is pointing here rather than at the high-voltage side.
- The turntable not turning — Last: a microwave heats unevenly by nature and relies on rotation to spread it.
Check in this order, and why that order
- Heat a cup of water on full power for a short burst — It is the standard test because it removes the load, the setting and your expectations from the question in one go.
- Check the power level and the cycle actually selected — A low power level cycles the output and feels like weak heating rather than none.
- Plug the machine directly into a wall socket with nothing else on the circuit — Extension leads and shared circuits change what this appliance can do.
- Close the door deliberately and listen for the latch — More than one switch has to make, and a door that needs a push has already told you something.
- Stop at the case — If water in the cavity does not heat with the supply proven and the door latching, the fault is on the high-voltage side and that is never owner work.
Before you open anything
Safety
Unplug the appliance or switch off its breaker before removing any panel. A capacitor can hold a charge after the power is off. Treat that as the first step of every check below that involves a panel, a grille or a cover.
Models in the B family — The numbers this page covers.
Reference| Model number | Family key |
|---|---|
| B20PXP78 | avanti:microwave:b |
When to call a technician
Settings, loads, sockets and the door latch are all owner checks on this machine. On these models the line falls at the outer case, and everything behind it. Call without working through anything at all for a socket or plug that heats under load, which is a fire risk rather than a nuisance.
Nearest pages of this class
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Experienced technician with 12 years specializing in washing machine and dryer repairs across all major brands.
