Microwave displays E4 and will not start; sometimes appears after a power surge or lightning storm.

Safety first: Unplug the microwave before inspection. Do not attempt to repair the high-voltage section yourself.

• Power cycle: Unplug the unit for 10–15 minutes to fully discharge the control board. Plug back in and check if E4 clears.
• Check outlet and surge protection: Test the wall outlet with another appliance. If you experienced a recent surge, consider using a surge protector for future use.
• Inspect power cord and connections: With the cabinet removed, inspect the power cord entry, line filter, and wiring to the control board for burns, loose connectors, or melted insulation. Tighten any loose spade connectors.
• Look for visible board damage: Examine the main PCB for burnt spots, bulging capacitors, or cracked solder joints. If any are present, the board should be replaced, not repaired, in most home settings.
• If no obvious wiring issue: The control board is likely defective and must be replaced with the correct Oster board for your model. This is typically a technician-level repair.