R-Pro › Field cases › Refrigeration systems

Defrost Termination Sensor Failure → Premature or Excessive Defrost

Likely causes

Thermistor short circuit → controller senses high temperature → premature defrost termination

Check: Measure sensor terminal resistance: ~6.8kΩ at +10°C, ~10kΩ at 0°C, ~16kΩ at -10°C. 0Ω = short circuit

What to do: Replace with same-spec NTC thermistor 10kΩ@25°C immediately → reinstall and verify operation

Thermistor open circuit → controller sees no temperature → defrost runs to timer maximum

Check: Measure sensor resistance: ∞ (infinite) indicates open circuit. Compare with reference sensor at same temperature

What to do: Replace immediately if open → reconfirm installation position (flush against evaporator outlet fins) → recheck backup timer 25 min

Sensor waterproof sheath damaged → moisture ingress causes irregular resistance fluctuation

Check: Visually inspect sensor sheath (cracks, wear) + verify if resistance fluctuates irregularly regardless of temperature

What to do: Apply self-amalgamating tape for temporary waterproofing → replace with same-spec sensor immediately → use fin clip for secure mounting on reinstall

Field tip

+10°C termination setpoint applies at the evaporator fins. If sensor is mounted at the drain pan, it reads 5–8°C higher than actual fin temperature, causing premature defrost cutoff.

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