Safety: Work on mains connections only if you are competent and local regulations allow. Otherwise, contact an electrician.

Check supply: Verify that the oven is on a dedicated circuit with the correct breaker rating. Avoid using extension cords or multi-plugs.
Terminal block inspection: Disconnect power, pull the oven out, and remove the rear cover to access the mains terminal block. Tighten all live, neutral, and earth screws. Look for heat discoloration or melted plastic; replace the block if damaged.
Measure voltage: With a multimeter and appropriate safety precautions, measure the voltage at the terminal block while the oven is running. If voltage drops significantly below the rated value, consult an electrician to correct the supply.
Filter and PSU check: Inspect the mains interference filter and the power supply section on the control board for burnt components. Replace the filter or board if visibly damaged.
Retest: After correcting supply issues and tightening connections, clear the error by power cycling and test a heating program.

Oven display shows E015, oven may shut down or not start

Possible Causes