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Error code reference

Samsung Air Conditioner Error Code E478

DC-link voltage error (under 150V or over 410V)

Samsung codesSamsung air conditioner codes

DC-link voltage error (under 150V or over 410V) - may display briefly after power cut.

First step from the service manual

Check power voltage and outdoor controller.

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

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What E478 means on a Samsung air conditioner

Error code E478 on a Samsung air conditioner indicates a DC-link voltage fault in the outdoor unit's inverter circuit. The DC-link is the high-voltage DC bus that sits between the rectifier stage and the inverter stage of the variable-frequency drive powering the compressor. This bus voltage must remain within a safe operating range to drive the compressor motor correctly. When the controller detects DC-link voltage falling below 150V or rising above 410V, it triggers E478 to protect the compressor and inverter components from damage.

The most common cause of the under-voltage condition (below 150V) is a loss or significant sag of incoming AC supply voltage, such as after a power cut, a brownout, or a tripped breaker. Samsung notes that E478 may appear briefly after a power interruption and clear on its own once supply voltage stabilizes. Over-voltage conditions (above 410V) can result from supply voltage spikes, a faulty filter capacitor on the DC bus that can no longer smooth rectified voltage, or a malfunctioning rectifier stage in the outdoor controller board.

Diagnosis begins at the power supply: verify that incoming line voltage at the outdoor unit is within the rated specification and stable. If supply voltage is normal and the error persists, attention shifts to the outdoor controller board itself, as an internal component failure in the rectifier or DC-link capacitor bank can produce out-of-range bus voltage even under correct input conditions. A failed or degraded electrolytic capacitor is a frequent culprit in over-voltage readings.

Source: manufacturer service documentation.

Common causes of E478

  1. 01

    Power interruption or brownout

    Wiring / connection

    A recent power cut, voltage sag, or brownout can cause the DC-link voltage to drop below 150V temporarily. The error often clears automatically once stable power is restored.

  2. 02

    Low or unstable supply voltage

    Wiring / connection

    If the incoming AC line voltage to the outdoor unit is consistently low due to a weak supply, undersized wiring, or a loose connection, the DC-link will read under-voltage. Measure supply voltage at the outdoor unit's terminal block under load to confirm.

  3. 03

    Failed or degraded DC-link capacitors

    Part replacement

    Electrolytic capacitors on the DC bus in the outdoor controller can deteriorate over time, losing their ability to maintain stable voltage. A failed capacitor can cause over-voltage readings above 410V. Capacitors showing bulging tops or electrolyte leakage confirm the fault.

  4. 04

    Faulty outdoor controller board

    Part replacement

    A failure in the rectifier stage or inverter circuitry of the outdoor controller board can produce incorrect DC-link voltage readings even when incoming power is normal. If supply voltage checks out and capacitors appear intact, the controller board is the next component to evaluate.

  5. 05

    Voltage spike or surge from the grid

    Wiring / connection

    A transient voltage spike entering through the supply line can push DC-link voltage above 410V and trigger E478. If this is occurring repeatedly, a surge protector or line conditioner installed upstream may help prevent recurrence.

See the test procedure for each cause

Frequently asked questions about E478

What does E478 mean on a Samsung air conditioner?
E478 indicates a DC-link voltage error in the outdoor unit's inverter circuit. The DC-link bus voltage has gone below 150V or above 410V, which falls outside the safe operating range. Samsung notes this code can appear briefly after a power cut and may clear on its own once stable power returns. If it persists, further diagnosis of the supply voltage and outdoor controller is needed.
Will resetting the air conditioner clear the E478 error?
A reset can clear E478 if the fault was caused by a temporary power interruption or brief voltage sag, as these conditions resolve once power stabilizes. To reset, power the unit off at the breaker, wait several minutes, then restore power. If the code returns after reset, the underlying cause such as low supply voltage or a faulty outdoor controller board must be addressed before the unit will operate normally.
Is it safe to keep running my air conditioner when E478 is displayed?
No, the unit will typically shut down when E478 is active because operating the compressor outside the safe DC-link voltage range can damage the inverter and compressor motor. Do not attempt to force the unit to run while the error is present. Address the root cause first, whether that is restoring stable power or repairing a faulty component, before putting the system back into service.
How do I fix E478 on my Samsung air conditioner?
Start by checking that the incoming AC supply voltage to the outdoor unit is stable and within the rated specification. If a power cut recently occurred, wait for power to fully restore and reset the unit at the breaker. If supply voltage is normal and the error persists, inspect the outdoor controller board for damaged capacitors or other visible component failures. A qualified technician with a multimeter can measure the actual DC-link voltage and confirm whether the board requires repair or replacement.
How much does it cost to repair a Samsung air conditioner showing E478?
Repair cost depends on the root cause. If the fix involves tightening a loose wiring connection or resetting after a power event, costs may be minimal, covering only a service call in the range of $75 to $150. If the outdoor controller board needs replacement, parts and labor typically range from $200 to $600 or more depending on the model and local labor rates. Getting a diagnostic visit first will clarify exactly which component has failed before committing to parts.

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