Smeg oven displays Er05 and will not start; timer and clock may still work.

Safety first: Disconnect the oven from mains power. Capacitors on the control board can retain charge; avoid touching bare conductors.

• Visual inspection: Remove the top or rear panel to access the main control board. Look for burnt areas, bulging capacitors, or darkened relay casings.
• Check wiring: Verify that the incoming live and neutral connections are tight and that the harness to the elements and fans is intact. Loose connections can cause arcing and board damage.
• Test supply voltage: With extreme caution and only if you are qualified, restore power and measure the mains voltage at the board input and the low-voltage outputs (e.g., 5 V or 12 V DC) feeding the display and logic. If low-voltage is missing, the board's power supply is faulty.
• Relay function: When a heating function is selected, you should hear a relay click. If not, and the board is powered, the relay or its driver circuit may have failed.
• Board replacement: Component-level repair is not recommended for most users. Replace the complete main control board with a genuine Smeg module, transferring any configuration jumpers as per service documentation.