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

Safety: Isolate power at the breaker or outdoor disconnect before removing any covers.

Steps:

• Open outdoor unit: Remove the top or side service panel to access the condenser coil and control board.
• Find condenser sensor: Look for a clip-on or inserted thermistor on the condenser coil tubing or near the compressor discharge line.
• Check attachment: Ensure the sensor is firmly clipped to the coil or pipe. If loose, reattach securely so it senses metal temperature.
• Inspect wiring: Look for insulation damage from heat or rubbing against the fan. Repair or replace damaged harness sections.
• Test sensor: Measure resistance with a multimeter and compare to the specified value for the current outdoor temperature. Replace if readings are open, shorted, or far off spec.
• Replace sensor: Install a genuine Westinghouse condenser temperature sensor and route wires away from fan blades and hot compressor surfaces.
• Check PCB: If the sensor is good and properly mounted but E4 remains, the outdoor PCB may be faulty and should be replaced by a professional.

Additional check: Ensure the condenser coil is clean and the outdoor fan is operating; overheating can also trigger related protection codes.