Heat pump dryer reports F21, poor drying performance, may stop with error mid-cycle.

Safety first: Disconnect mains power before accessing internal components.

• Clean heat exchanger: Ensure the evaporator and condenser surfaces are clean and not blocked by lint, as this can cause abnormal temperatures and pressures.
• Locate sensors: Identify the NTC sensors on the heat pump circuit (evaporator and condenser) and any pressure sensor on the sealed system. Inspect wiring and connectors for damage or looseness.
• Resistance check: Test NTC sensors for correct resistance at room temperature. Replace any that are open, shorted, or significantly out of specification.
• Technician diagnosis: If sensors and wiring are good, a sealed system or pressure issue may be present. A qualified technician should check pressures and compressor performance.