The short answer
Here is what the symptom means on a Black+Decker microwave: a machine that heats and then stops is describing a protective device, and protective devices open for a reason. Clearing the vents is the actual repair in most of these; if it keeps cutting out with clear airflow, that is a fault rather than a setting.
What you actually see
- The oven runs a full cycle and the food is cold — Turntable turning, light on, fan running, nothing warm — the classic no-heat picture.
- A loud buzz during the cycle with no heat — A buzz under load with no result is a component drawing current and doing nothing useful.
- Heat stops part way through a long cycle — Stopping mid-cycle points at a thermal device rather than at a permanent failure.
Causes, ranked for this microwave
- Blocked ventilation causing repeated cut-outs — Ranks first because a countertop oven pushed into a corner or under a shelf recirculates its own hot air and trips its protection repeatedly. This is the entry most often blamed correctly and diagnosed carelessly, so take the extra minute.
- Thermal cut-out opened after overheating — Second because the pattern is a machine that heats, stops heating, and works again after standing — which no permanent failure does. Confirm it before moving on, because the entries below read differently once it is ruled out.
- Door latch not fully closing its switches — Third because a microwave will not produce heat unless every door switch confirms the door is shut, and a worn latch closes some of them and not others. Rule it in or out properly here; a maybe at this position costs an hour further down.
What to check, in order
- First — the vents around the case — Feel the air leaving the vents during a cycle. Hot air with nowhere to go is the whole problem.
- Second — whether it works again once cool — Let the oven stand unplugged for an hour, then repeat the water test. Heat that returns points at ventilation.
Checks an owner can do safely
- Step 1. Stop at any smell or spark — A burning smell or visible sparking means unplugging the oven immediately. Neither is a symptom to keep testing through.
Tools: noneAbout 1 min
- Step 2. Run the cup-of-water test — Put a mug of cold water in the centre of the turntable and run a short cycle at full power. Feel the water afterwards. This removes every ambiguity about whether the oven heats.
Tools: mug, waterAbout 5 min
- Step 3. Close the door firmly and listen — Press the door properly shut and listen for a clean double click before starting. A start that only works under pressure convicts the latch.
Tools: noneAbout 5 min
Before you open anything
Safety
Unplug the appliance or switch off its breaker before removing any panel. The high-voltage capacitor in a microwave can hold a lethal charge long after the cord is out, and it is not discharged by waiting. Nothing inside the outer cabinet is an owner-serviceable part, and no fault justifies running the oven with the case open or with a door switch held closed by hand.
When this becomes a service call
Stop and call when the next thing to do meets any of these. Any burning smell, visible spark or breaker trip is a stop-now condition rather than a step in the diagnosis.
Sources
General guidance- The Control of Hazardous Energy (Lockout/Tagout) — OSHA 1910.147 and manufacturer service manuals, fetched
Nearest pages
What this page does not cover
One limitation worth stating up front: this page is written for the Black+Decker microwave range rather than for your particular unit. It does not name families, model numbers or control revisions. Where a page exists that does, it is more precise than this one and it is linked under Related pages.
Lead Appliance Repair Technician · 12 years experience
Experienced technician with 12 years specializing in washing machine and dryer repairs across all major brands.
