Air fryer displays E04 and refuses to start or stops mid-cycle; temperature readings seem incorrect or fluctuate rapidly.

Safety first: Disconnect the Bosch air fryer from power and allow it to cool completely.

Diagnosis steps:

• 1. Power reset: Unplug for 5 minutes, then reconnect and try a short cycle. If E04 reappears immediately, the sensor circuit is likely faulty.
• 2. Access sensor: Remove the top or rear cover to locate the NTC temperature sensor, usually clipped to the heater plate or mounted in the hot air channel.
• 3. Inspect wiring: Check the thin sensor wires for cuts, melted insulation, or pulled connectors. Reseat the plug on both the sensor and the control board.
• 4. Test sensor resistance: Disconnect the sensor and measure resistance with a multimeter. At room temperature, an NTC typically reads between 5 kΩ and 200 kΩ depending on type. A reading of 0 Ω (short) or OL (open) indicates a failed sensor.
• 5. Check mounting: Ensure the sensor tip is firmly in contact with the metal surface or in its designated well. A loose sensor can cause erratic readings and trigger E04.

Repair: Replace the NTC sensor with a Bosch-compatible part of the same resistance curve and temperature rating. Route the harness away from the heating element to avoid future heat damage.