Air fryer displays E4 and will not start a cooking program.

Safety first: Unplug the Ultima Cosa air fryer. Wait until the unit is completely cool before opening any panels.

• Clean and inspect: Remove basket and drawer. Clean heavy grease from the heating element area and top interior. Check that air inlets and outlets are not blocked.
• Access thermal devices: Remove the rear or top cover to access the heating assembly. Locate the thermal fuse and/or high-limit thermostat mounted on or near the heater bracket.
• Test continuity: With a multimeter, check continuity across the thermal fuse and thermostat. A good fuse/thermostat will show near-zero ohms. If the thermal fuse is open (no continuity), it has blown and must be replaced with an identical temperature and current rating.
• Replace thermal fuse: Cut out the old fuse and crimp in the new one using high-temperature crimp connectors. Do not solder directly to the fuse, as heat can damage it. Ensure it is mounted in the same location and orientation.
• Check for root cause: Verify that the fan runs properly and that vents are clear to prevent repeat overheating. If the unit repeatedly blows fuses, the control board may be overdriving the heater and should be checked.

If you are not experienced with electrical repairs, have a professional replace the thermal fuse and inspect the heater circuit.