Display shows “Err4” and the unit will not operate.

Safety first: Unplug and let the unit cool completely.

• Allow full cool-down: Some Fellow heating bases have self-resetting thermal protection. Leave unplugged for at least 30–60 minutes, then retry with water filled to the minimum line.
• Check for scale: Heavy limescale on the heating plate can cause localized overheating. Descale thoroughly using a 1:1 vinegar and water solution as per Fellow’s instructions.
• Inspect for discoloration: Look at the underside of the kettle or brewer for burn marks or warping, which indicate repeated dry-boil events.
• If Err4 returns quickly: The internal thermal cutoff or boil-dry thermostat may be weakened and tripping prematurely. Replacement requires opening the base and swapping the thermostat or thermal cutoff assembly.