Rheem inverter air conditioner displays E4 and may shut down in heating mode or during defrost cycles.

Safety first: Turn OFF all power to the outdoor unit. The discharge line can be very hot; allow time to cool before touching.

• Locate the sensor: Open the outdoor unit panel and find the thermistor attached to the small-diameter copper line leaving the compressor (discharge line).
• Check attachment: Ensure the sensor is tightly clamped to the pipe with the original clip or metal strap. Re-secure if loose.
• Inspect for overheating: Look for discoloration or melted insulation around the sensor and wiring. Replace the sensor if heat-damaged.
• Test resistance: Disconnect the thermistor and measure resistance. Compare to Rheem’s chart. Replace if open, shorted, or out of spec.
• Investigate root cause: If the sensor is repeatedly failing, there may be a deeper issue (overcharge, blocked airflow, non-condensables). A licensed technician should check system pressures and operation.

Warning: Do not cut or open refrigerant lines. Discharge line temperatures can exceed safe touch limits during operation.