Bosch Microwave: Replace High-Voltage Capacitor

Read the safety warning first#

The HV capacitor stores a potentially lethal charge and can hold 2000+ volts even after the microwave is unplugged. You MUST discharge it before removal — a shock from a charged microwave capacitor can be fatal. If high-voltage work is outside your comfort zone, stop here and call a technician.

Is the capacitor actually the fault?#

• Open (most common): the microwave runs but produces zero heat — the same symptom as a bad diode, so test both.
• Shorted: loud buzzing or humming from the transformer area, often with the line fuse blowing as the transformer forces current through the short.
• Intermittent: works sometimes, or heats weakly and inconsistently, from internal damage that fails under load.

If the fuse blows the instant you press start, suspect a shorted capacitor or the door monitor switch. If it runs with no heat and no other clue, test the capacitor and the diode.

Tools, part, and safety#

• Tools: insulated-handle screwdriver (for discharge), Torx T15 and T20 drivers, a multimeter with a capacitance range, needle-nose pliers, a 1/4-inch nut driver, and safety gloves
• Part: HV capacitor matching the spec — about 0.95uF (some models 1.0uF or 1.05uF) rated 2100VAC minimum. The Bosch number is model-specific (commonly BSH 00614735); an aftermarket equivalent is fine if the ratings match.
• Safety: unplug the unit fully before anything else.

Step-by-step#

1. Access the capacitor#

Unplug the microwave. On built-in Bosch models (BFL series) remove it from the cabinet (four Torx T20 screws); on countertop models take off the outer shell (side and rear screws, slide it back). The HV capacitor is the largest capacitor inside — a metal cylinder about 3-4 inches long, near the transformer.

2. Discharge it — non-negotiable#

Before touching any component:

1. Identify the two spade terminals on top of the capacitor.
2. Put one hand behind your back so current cannot cross your chest.
3. With an insulated-handle screwdriver, bridge both terminals at once with the metal shaft.
4. Hold for about 3 seconds — a spark is the stored energy releasing.
5. Verify with the multimeter on DC volts: it must read 0V. If any voltage remains, discharge again.

A gentler alternative is a 10kOhm 10-watt resistor on insulated clip leads bridged across the terminals for a controlled, sparkless discharge.

3. Remove the old capacitor#

Note the wiring: one spade goes to the HV diode, the other to the transformer secondary, and there may be a bleeder resistor clipped across the terminals (save it if the new part lacks one). Pull each spade connector straight off with needle-nose pliers — no side-to-side wiggle, which bends the terminal. Undo the mounting bracket bolt or strap and lift the capacitor out.

4. Confirm it failed#

On a capacitance meter a good part reads about 0.90-1.10 uF; an open one reads zero or OL. With an analog ohmmeter on its highest range, a healthy capacitor deflects then drifts back toward infinity, a shorted one stays at zero, and an open one never moves. A bulging top or oil weeping from the base is catastrophic failure.

5. Fit the new capacitor#

Check the replacement: capacitance around 0.95uF (within about 10% is fine), voltage 2100VAC or higher, matching physical size and spade terminals. Mount it in the bracket in the original orientation and secure it. Polarity does not matter on these AC capacitors — either wire to either terminal. If the original had a bleeder resistor, move it to the new one.

6. Reassemble and test#

Before closing up, confirm no loose wires or stray tools and recheck every HV connection: transformer-to-capacitor, capacitor-to-diode, diode-to-ground, and the magnetron leads. Refit the shell or reinstall in the cabinet, then heat a cup of water for 60 seconds at full power — it should come out noticeably warm. Any abnormal buzzing points to a connection or the transformer.

Troubleshooting#

• Still no heat: test the HV diode and magnetron — a capacitor can fail secondary to them.
• Loud hum now: a lead is loose or the capacitor is vibrating against the chassis; reseat and secure it.
• New capacitor bulges within weeks: the magnetron is arcing and drawing excess current, destroying downstream parts — replace the magnetron.
• Line fuse blows after the swap: the capacitor may be the wrong type, or another HV component is shorted — recheck the diode and magnetron.

When to call a professional#

Get help if HV capacitor work is outside your comfort zone, if the transformer grinds (its own laminations failing), if several HV parts failed together (a cascade), or if the mounting area shows fire or melting.