mypros+
Error code reference

LG Hvac Error Code 21

Inverter compressor high amperage.

LG codesLG hvac codes

Inverter compressor high amperage.

First step from the service manual

Verify proper 208/240V AC feeding unit, ensure unit is not operating in high-head situation, verify 3 compressor windings all equal at 1-4Ω, verify ∞Ω to ground, check amperage. Verify system is not undercharged. Weigh out charge with electronic scale & calculate lineset length.

The complete diagnostic procedure includes additional test steps, resistance specifications, wiring diagram references, and component test points.

MyPros+ Max

Get the full diagnostic procedure for 21.

  • Step-by-step tests with expected resistance, voltage, and continuity values
  • Wiring diagram references and connector pinouts from the OEM service manual
  • Verified part numbers when replacement is required
Get the full procedure

$29/month · Cancel anytime · Or $9.95 3-day pass

What 21 means on a LG hvac

Error code 21 on an LG inverter-driven HVAC system indicates the inverter compressor is drawing excessive amperage during operation. The inverter drive board monitors current to the compressor in real time, and when amperage climbs beyond the acceptable threshold, it triggers this fault to protect the compressor and drive circuitry from thermal or electrical damage. This is a critical protection fault that will shut down the compressor until the underlying cause is resolved.

The most common electrical root causes fall into two categories: supply voltage issues and compressor winding integrity. The system requires a stable 208/240V AC supply, and voltage that is too low forces the compressor to draw higher current to maintain the same workload, quickly pushing amperage out of spec. On the compressor side, all three motor windings should measure equally in the 1 to 4 ohm range with infinite resistance to ground. A winding that has drifted out of balance or shows continuity to ground points to internal compressor failure.

Refrigerant charge is equally important in this diagnosis. An undercharged system causes the compressor to work harder and run hotter, which directly increases current draw. Before condemning any component, the refrigerant charge should be verified by weighing the refrigerant out using an electronic scale and accounting for lineset length in the charge calculation. High-head pressure conditions, caused by blocked condenser coils, inadequate airflow, or overcharge, also force elevated amperage and must be ruled out during the diagnostic process.

Source: manufacturer service documentation.

Common causes of 21

  1. 01

    Low or unstable supply voltage

    Wiring / connection

    If the unit is not receiving proper 208/240V AC, the compressor must draw higher current to compensate for the reduced voltage. Measure voltage at the unit's terminal block under load to confirm stable supply.

  2. 02

    Incorrect or insufficient refrigerant charge

    Wiring / connection

    An undercharged system causes the compressor to work harder, increasing amperage. Weigh out the refrigerant charge using an electronic scale and factor in lineset length to verify the charge is within specification.

  3. 03

    High-head pressure condition

    Wiring / connection

    Blocked condenser coils, restricted airflow, or a non-condensable in the system can cause elevated discharge pressure, forcing the compressor to pull excessive current. Inspect the condenser coil and verify adequate airflow around the outdoor unit.

  4. 04

    Degraded or failed compressor windings

    Part replacement

    All three compressor motor windings should measure 1 to 4 ohms with infinite resistance to ground. A winding reading outside this range or showing any continuity to ground indicates internal compressor failure requiring replacement.

  5. 05

    Faulty inverter drive board

    Part replacement

    If supply voltage, refrigerant charge, head pressure, and compressor winding resistance all check out within spec, the inverter board itself may be misreading current or generating an incorrect drive signal. Confirm all other causes are ruled out before replacing the board.

See the test procedure for each cause

Frequently asked questions about 21

What does 21 mean on a LG hvac?
Error code 21 indicates the inverter compressor is drawing excessive amperage. The system's inverter drive board detected current above the acceptable operating threshold and shut down the compressor to prevent damage. The cause can range from low supply voltage and incorrect refrigerant charge to a high-head pressure situation or failing compressor windings. A full electrical and refrigerant system diagnosis is required to resolve it.
Can I fix error code 21 on my LG HVAC myself?
Some of the preliminary checks, like verifying the circuit breaker, inspecting the outdoor unit for blocked airflow, and confirming the condenser coil is clean, can be done by a homeowner. However, checking supply voltage under load, measuring compressor winding resistance, and verifying refrigerant charge using an electronic scale require proper tools and, in the case of refrigerant work, EPA certification. Most homeowners will need a licensed HVAC technician to complete this diagnosis.
Is it safe to keep running my LG HVAC with error code 21 active?
No. Error code 21 is a protective fault, meaning the system has already shut down the compressor to prevent damage. Attempting to force the system to run while this fault is active risks damaging the inverter drive board and the compressor itself, both of which are expensive components. The unit should remain off until the underlying cause is diagnosed and corrected.
Will resetting the unit clear error code 21?
A power reset may temporarily clear the fault and allow the system to restart, but if the underlying cause is not corrected, the error will return quickly once the compressor begins drawing excessive current again. Repeated resets without fixing the root cause can accelerate wear on the compressor and inverter board. A proper diagnostic must be performed to permanently resolve the fault.
How much does it cost to repair a LG HVAC showing error code 21?
Repair costs vary significantly depending on the root cause. If the issue is a low refrigerant charge, expect to pay for diagnostic labor plus refrigerant, typically in the range of $150 to $400. If the compressor windings have failed and the compressor requires replacement, costs can range from $800 to $2,000 or more depending on the unit size and labor rates in your area. An inverter drive board replacement generally falls somewhere in between, often in the $300 to $900 range including parts and labor.

Related LG hvac codes

All LG hvac codes
View all 75 LG hvac codes

78,000+ service manuals · 160,000+ error codes · 55+ brands

Get Started

$29/month · Cancel anytime · Or $9.95 3-day pass