Unit stops and displays E4, often during heating mode on heat pump models

Safety: Disconnect power before opening any panels. The discharge line can be very hot during operation.

Steps to address E4:

• Allow unit to cool: If the unit just shut down, wait 10–15 minutes for components to cool before touching any copper lines.
• Access compressor compartment: Remove the outdoor or side panel to expose the compressor and copper discharge line.
• Locate discharge sensor: The thermistor is typically strapped or clipped to the small-diameter copper line leaving the compressor.
• Check attachment: Ensure the sensor is tightly secured to the discharge line with the original clip or metal strap. A loose sensor can misread temperature and trigger E4.
• Inspect wiring: Look for burned, brittle, or melted insulation near the compressor. Repair or replace damaged wiring and keep it away from hot surfaces.
• Clean coils and filters: Dirty indoor or outdoor coils and clogged filters can cause high discharge temperatures. Clean the air filter, indoor evaporator coil, and outdoor condenser coil with a soft brush and appropriate coil cleaner.
• Test sensor: With power off, disconnect the thermistor and measure resistance. Compare to the service chart. Replace if out of range.
• Refrigerant issues: If E4 recurs after sensor and airflow checks, the system may have a refrigerant problem (overcharge, undercharge, or restriction). This requires an EPA-certified technician.

Do not: Attempt to adjust refrigerant charge yourself. This is specialized work and may be illegal without proper certification.