Bosch Microwave ER4: Severe Overheating, So Stop Using It

What ER4 means

ER4 means the microwave detected heat far beyond normal and is treating it as a serious hazard. Unlike most codes, this one is not about convenience. Heat may be building near the magnetron or in the cavity, and the safeguards that should have prevented it may not be working. The right response is to stop cooking immediately and cut power, not to wait and see whether it clears.

Common causes of ER4

Magnetron running hot or failing
Vents blocked so heat cannot escape
Thermal protection part failed

✓ One safe check you can try

1. Stop the cycle and do not restart it. 2. If it is freestanding, unplug it; if built-in, switch it off at the breaker. 3. Keep the door closed and leave the area clear of anything flammable. 4. Do not run it again. If you smelled burning or saw smoke, call the fire department first, then call us.

! Why to stop there

Running a unit with failed thermal protection can start a fire in a kitchen.
A hot magnetron can fail violently, and inspecting it yourself is dangerous.
Even a code that clears can hide a part that is no longer protecting the oven.

Opening a panel yourself does not automatically cancel a manufacturer warranty, but it can cause accidental damage, and some manufacturers set their own service conditions. Check your warranty terms, or ask us, before deciding.

→Same appliance

Other Microwave codes

E1Bosch Microwave E1: Thermistor Circuit Reads Open E4Bosch Microwave E4: The Thermistor Circuit Reads Shorted E-04Bosch Microwave E-04: Over-the-Range Humidity Sensor Not Reporting E-11Bosch Microwave E-11: Over-the-Range Key Read as Held for a Minute E2Bosch Microwave E2: The Safety Shutoff Stopped Your Cycle E3Bosch Microwave E3: Door Will Not Latch or Release Properly E11Bosch Microwave E11: Moisture Detected at the Control Panel E17Bosch Microwave E17: The Fast Preheat Did Not Finish

ER4ER4 questions

Can I try ER4 once more to see if it clears?

We strongly advise against it. The code signals possible fire risk, and the safeguard that normally stops overheating may not be reliable. Keep the unit off until we inspect it.

Could a blocked vent alone cause ER4?

It can add to it, particularly in a built-in cabinet. But a blocked vent does not rule out a failed protective part, so inspection is still needed before use.

Is it worth repairing after ER4?

Sometimes, sometimes not. If the magnetron or protection needs replacing, we give a written price so you can compare with a new unit. Repairs carry a 30-day warranty.