Indoor unit stops, E5 error, often during heating mode changeover

Safety first: Isolate power to the outdoor unit. The discharge line can be extremely hot during operation; only work on a cooled system.

• Locate the sensor: Open the outdoor electrical/coil compartment. The discharge pipe thermistor is usually clipped or strapped to the small-diameter copper line leaving the compressor.
• Check attachment: Ensure the thermistor is firmly attached with its clip or metal strap and has good contact with the pipe. Re-secure if loose.
• Inspect wiring: Follow the sensor wires back to the PCB. Look for chafing against metal edges or damage from UV/rodents. Repair or replace as needed.
• Resistance check: With the sensor unplugged and system cool, measure resistance and compare to Fujitsu's temperature-resistance chart. Replace the thermistor if readings are open, shorted, or unstable.
• Test run: After replacement, restore power and test in heating and cooling to confirm E5 is cleared.
• PCB diagnosis: If a known-good sensor still triggers E5, the outdoor PCB input may be defective and should be replaced by a professional.