Thermador Microwave ER4 Error: Extreme Overheat Detected
Thermador microwave Er4 error indicates extreme overheat detected. Stop using the appliance and call a Thermador-certified technician immediately for diagnosis. What Does Thermador Microwave Error Code Er4 Mean? The Er4 error code on a Thermador Professional or Masterpiece built-in microwave means the appliance detected extreme overheating inside the cooking cavity. This is the most serious […]
Quick Assessment
Answer to continue safely
Is it safe to keep using?
No. Do not use the microwave after Er4 until the cavity has been inspected and any burn damage has been assessed by a technician. If the cavity walls, waveguide cover, or magnetron show damage, the unit needs professional service before it can be used safely.
Can I reset the code?
Yes. If Er4 was triggered by a one-time event (foil left in the cavity, food overcooked), a complete cool-down followed by a breaker reset can clear the fault. If Er4 is caused by a failing magnetron, it will return on the next use.
When to stop immediately?
Stop if you notice: Visible flames or persistent smoke inside the cavity, Er4 returns on the next use after the cavity cools completely.
Symptoms You May Notice
Microwave shuts down abruptly during cooking with Er4
The unit cuts power to the magnetron mid-cycle, the display freezes on Er4, and the door cannot be opened immediately while the unit cools.
Smoke, sparks, or burning smell from inside the cavity
Visible smoke or the odor of burning food or plastic accompanies the Er4 shutdown, indicating an actual overheating or fire event inside the cavity.
Cavity temperature feels extremely hot to the back of the hand near the vent
The exhaust vents on the top or back of the Thermador built-in microwave emit noticeably hot air, confirming the cavity reached a dangerously high temperature.
Possible Causes
Combustible material inside the cavity during operation
Foil, metallic trim on dishware, or improperly wrapped food ignited inside the cavity, causing the temperature to spike far above normal operating range.
DIY PossibleMicrowave power level set far too high for the food type
A dense or very small item was cooked at maximum power for an excessive duration, causing localized superheating that tripped Er4.
DIY PossibleFailed magnetron running at uncontrolled power
A failing magnetron stuck in a continuous-on state due to a faulty interlock or relay caused cavity temperature to rise uncontrollably until Er4 triggered the thermal cutoff.
Requires ProfessionalSafe Checks You Can Do
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1
Do NOT open the door immediately — wait for the unit to cool
If Er4 appears during cooking, do not attempt to open the microwave door right away. Wait a minimum of five minutes for the cavity to cool. If you see smoke or flames through the window, do not open the door — a closed door starves a small fire of oxygen. If smoke persists, disconnect power at the breaker.
Opening a microwave door during an internal fire event feeds oxygen to the cavity. On Thermador Professional built-in microwaves the door latch holds while the unit is in a thermal fault state — this is intentional safety design.
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2
Inspect the cavity for damage before attempting any use
After the unit cools completely, open the door and inspect the interior walls, turntable, and waveguide cover for burn marks, charring, or melted surfaces. Wipe away any food residue with a damp cloth. Do not use the microwave if the waveguide cover or cavity walls show burn damage.
A scorched waveguide cover must be replaced before the next use — carbon deposits on the cover can arc during the next cooking cycle and cause a new Er4 event or a fire.
When to Call a Professional
Contact a qualified technician if:
- Burn marks or charring on the waveguide cover or cavity walls
- Magnetron shows signs of arcing or physical damage on visual inspection
- Er4 reappears immediately on the next cooking attempt after cavity inspection
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