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Error code reference

LG Heat Pump Error Code E321

SUMP TEMP DIGITAL sensor error OPEN/SHORT (detected when outdoor temp over -10°C; over 4.95V/-30°C or below 0.5V/151°C).

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What the E321 error code actually means

Your LG heat pump is showing an error because the temperature sensor located near the bottom of the compressor is sending a reading that is either impossibly cold or impossibly hot, which tells the system something is wrong with that sensor or its wiring. The heat pump has shut down to protect itself from running without accurate temperature information. A technician will need to check the sensor wiring and the sensor itself to get the system running again.

Diagnostic procedure

The full E321 procedure

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1
First diagnostic step
Check if sensor connector is separated from PCB; measure resistance across terminals.
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Most common causes of E321

Ranked by how often they cause this code
  1. 1Disconnected sensor connector
  2. 2Failed NTC thermistor (open circuit)
  3. 3Shorted sensor or wiring
  4. 4Damaged or corroded sensor wiring harness
  5. 5Faulty outdoor unit PCB

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Questions people ask about E321

What does E321 mean on a LG heat_pump?
E321 indicates that the sump temperature sensor circuit on the outdoor unit has been detected in an open or short condition. The system expects a signal voltage between 0.5V and 4.95V, corresponding to temperatures between 151°C and -30°C. When the signal falls outside that window while outdoor temperatures are above -10°C, the unit logs E321 and stops operating to protect the compressor.
Is it safe to keep running my heat pump with an E321 error?
No. The sump temperature sensor protects the compressor from operating under dangerous thermal conditions. Running the unit without a functioning sump sensor removes that protection and risks compressor damage, which is one of the most expensive repairs on a heat pump. The system should remain off until the fault is diagnosed and corrected.
How much does it cost to repair an E321 error on an LG heat pump?
Repair costs vary depending on the root cause. If only the sensor connector needs re-seating, the cost may be limited to a service call fee, typically in the range of $75 to $150. Replacing the sump temperature sensor itself usually falls in the $100 to $300 range including parts and labor. If the outdoor PCB is at fault, replacement costs can range from $300 to $700 or more depending on the model.
Can I fix the E321 error myself?
A homeowner can safely check whether the sensor connector has simply become unplugged from the outdoor PCB, and re-seating it may resolve the error. However, measuring sensor resistance, tracing wiring faults, or replacing the sump sensor or PCB requires working inside the outdoor unit and should be handled by a qualified technician familiar with refrigeration equipment.
Will resetting the heat pump clear the E321 error?
A power cycle may temporarily clear the error display, but if the underlying sensor or wiring fault still exists, E321 will return as soon as the outdoor temperature climbs above -10°C and the control board re-evaluates the sensor signal. A reset alone does not fix the hardware problem causing the fault.

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