Outdoor Thermistors Fault

What does 6-time blink 2.5-second OFF mean?

This fault occurs when any outdoor thermistor (e.g., ambient, discharge, liquid line) experiences a short or open circuit during compressor operation. This prevents the outdoor unit from accurately sensing temperatures, impacting defrost cycles, capacity control, and overall system efficiency. Failure to address this can lead to system instability, freezing, or inefficient heating/cooling.

• Outdoor ambient thermistor (RT1, RT2), discharge thermistor (RT3), or liquid line thermistor (RT4) wiring harness damaged or corroded, creating an open circuit or short.
• Water ingress or extreme weather conditions causing corrosion or shorting at the thermistor sensor head or connector.
• Internal failure of one or more outdoor thermistors (e.g., element degradation, cracked housing) leading to incorrect resistance readings.
• Loose or poor connection at the thermistor input terminals on the outdoor control P.C. board (e.g., J503, J504, J505).
• Electrostatic discharge (ESD) or power surge damaging the thermistor input circuitry on the outdoor P.C. board.

Click steps to track your progress.

1. 1

   Disconnect main power to the outdoor unit.

2. 2

   Identify and disconnect all outdoor thermistors (e.g., ambient RT1, RT2; discharge RT3; liquid line RT4) from the outdoor P.C. board.

3. 3

   Measure the resistance of each thermistor individually using a multimeter. Compare the measured resistance to the manufacturer's specified temperature-resistance curve (e.g., 15kΩ at 25°C).

4. 4

   Inspect the wiring harnesses for each thermistor for physical damage (cuts, abrasions), corrosion, or loose connections. Check continuity from the thermistor to its P.C. board connector pins.

5. 5

   Clean thermistor connectors and the corresponding P.C. board receptacles using electrical contact cleaner. Ensure proper seating upon reconnection.

6. 6

   Replace any thermistor that shows an open circuit, short circuit, or resistance value significantly outside of its specified range. Restore power and monitor operation.