Outdoor unit or window unit shows E3 and cooling stops intermittently, often during hot weather.

Safety: Turn off power at the outdoor disconnect or main breaker. Wait several minutes for capacitors to discharge.

Steps:

• Access outdoor section: Remove the service panel on the outdoor unit (for split systems) or rear/side cover on a window unit to expose the control compartment.
• Locate ambient sensor: The outdoor ambient thermistor is usually mounted near the air intake grille or on the condenser coil frame, with two wires leading to the outdoor PCB.
• Visual inspection: Check for sun damage, broken wires, rodent damage, or corrosion at the connector. Clean light corrosion with electrical contact cleaner.
• Check connection: Reseat the sensor connector on the PCB. Ensure the wire is not pinched by the cabinet.
• Measure resistance: Use a multimeter to check the thermistor resistance and compare to the Westinghouse temperature-resistance chart. Replace if open, shorted, or far out of spec.
• Replace sensor: Install the correct Westinghouse outdoor ambient sensor, securing it in the same location away from direct sunlight and hot discharge air.
• PCB diagnosis: If E3 persists with a known-good sensor, the outdoor control PCB may be defective and should be replaced by a qualified technician.

Warning: Do not work inside the outdoor unit with power on; the fan and compressor can start unexpectedly.