Unit stops and displays E4; may occur during heating or cooling, often after running for some time.

Safety first: Turn off power at the breaker. The discharge line can be extremely hot; allow the unit to cool before touching any piping.

Steps:

• Check airflow and cleanliness: Before assuming a sensor fault, inspect the outdoor coil and fan. Clean any heavy dirt, leaves, or obstructions that could cause high discharge temperatures.
• Locate T3 sensor: On the outdoor unit, find the sensor clipped to the compressor discharge pipe (small copper tube leaving the compressor). Ensure it is firmly attached with thermal paste or clip as originally installed.
• Inspect wiring: Follow the sensor wires back to the PCB and confirm the connector is secure and undamaged.
• Test sensor: With the unit cool and power off, disconnect the sensor and measure resistance. Compare to Blaupunkt’s NTC specification. Replace if readings are out of range.
• Check for systemic issues: If the sensor is good but E4 occurs frequently, the system may be overheating due to low refrigerant charge, non-condensables, or a failing outdoor fan motor. These require a licensed refrigeration technician.