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Error code reference

LG Hvac Error Code 57

Communication error between outdoor main PCB and inverter PCB.

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Communication error between outdoor main PCB and inverter PCB.

First step from the service manual

Check wiring & fuses; verify LEDs on both PCBs are lit. If OK, either inverter or main PCB defective.

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

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What 57 means on a LG hvac

Error code 57 on an LG HVAC system signals a communication failure between the outdoor unit's main PCB and its inverter PCB. These two boards must continuously exchange data to coordinate compressor speed, frequency control, and protection functions. When the signal between them is interrupted or corrupted, the system cannot safely operate the inverter-driven compressor and shuts down, triggering this fault.

The most common entry point for diagnosis is the wiring harness that connects the two boards. Loose connectors, damaged wire insulation, corrosion at terminal pins, or a blown fuse on the communication circuit can all break the data link. Technicians should inspect both the wiring and any inline fuses on the outdoor unit before concluding that a board has failed. LED indicators on both PCBs provide a quick status check: if a board is not powered or is internally faulted, its status LED will be off or blinking in an abnormal pattern.

If the wiring and fuses check out and both boards appear to be powered normally, the fault points to a defective inverter PCB or a defective main PCB. In some cases the inverter PCB fails due to power transistor or driver circuit damage, while the main PCB can fail due to component-level faults affecting its communication port. Replacing the faulty board is the corrective action, and determining which board is at fault may require substitution testing or detailed voltage checks at the communication terminals.

Source: manufacturer service documentation.

Common causes of 57

  1. 01

    Loose or damaged wiring harness

    Wiring / connection

    The ribbon or wire harness connecting the main PCB and inverter PCB can work loose from its connectors or suffer insulation damage from heat or vibration. Inspect all connectors between the two boards and reseat any that are loose or corroded.

  2. 02

    Blown communication circuit fuse

    Part replacement

    A fuse protecting the communication circuit on the outdoor unit can open due to a surge or short, cutting the data link between the boards. Locate and test all fuses in the outdoor unit and replace any that have failed.

  3. 03

    Defective inverter PCB

    Part replacement

    The inverter PCB can fail internally, often due to damage to driver circuits or power components, making it unable to send or receive communication signals. If wiring and fuses are intact and the board's status LED is off or abnormal, inverter PCB replacement is likely needed.

  4. 04

    Defective outdoor main PCB

    Part replacement

    The main PCB can develop a fault on its communication port or related circuitry, preventing it from exchanging data with the inverter board. If the inverter PCB tests normal and wiring is confirmed good, the main PCB should be suspected.

  5. 05

    Corrosion at PCB connectors

    Wiring / connection

    Moisture intrusion into the outdoor unit can cause oxidation at the board-to-board connector pins, increasing resistance and degrading the communication signal. Clean or replace affected connectors and address any gaps in the unit's weatherproofing.

See the test procedure for each cause

Frequently asked questions about 57

What does 57 mean on a LG hvac?
Error code 57 indicates a communication failure between the outdoor unit's main PCB and its inverter PCB. These two boards must exchange data continuously to control compressor operation, and when that link breaks down the system faults out and shuts off. The cause can range from a loose wire or blown fuse to a fully failed circuit board.
Can I fix error code 57 on my LG HVAC myself?
A knowledgeable DIYer can safely check for obvious wiring issues and inspect fuses in the outdoor unit, but working inside the outdoor unit on inverter components involves high-voltage DC bus capacitors that can hold a dangerous charge even after power is disconnected. If basic wiring checks do not resolve the error, PCB diagnosis and replacement should be handled by a qualified technician. Attempting to swap inverter or main PCBs without proper training carries a serious shock risk.
Will resetting my LG HVAC clear error code 57?
A power reset may temporarily clear the fault display, but if the underlying wiring fault or PCB failure is still present the error will return as soon as the system attempts to restart and the boards try to communicate. A reset is useful for confirming whether the fault is intermittent, but it does not fix the root cause. Persistent or recurring error 57 requires physical inspection and component testing.
Is it safe to run my LG HVAC with error code 57 active?
No. When error code 57 is active, the system has already shut down the compressor because safe coordinated control is not possible without communication between the two PCBs. Attempting to force the unit to operate in this state is not recommended and could cause further damage to the inverter or compressor. Leave the unit off until the fault is diagnosed and repaired.
How much does it cost to repair error code 57 on an LG HVAC?
Repair cost depends on what component has failed. If the fix is a loose connector or a fuse, parts cost is minimal and labor may be one to two hours. If the inverter PCB or main PCB needs replacement, parts alone typically range from $150 to $600 or more depending on the model, plus labor. Getting a diagnosis from a certified technician before purchasing parts is the best way to avoid unnecessary expense.

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