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Error code reference

Samsung Air Conditioner Error Code P4

Reverse phase or open phase error

Samsung codesSamsung air conditioner codes

Reverse phase or open phase error - 3-phase power phase order is reversed or one or more phases are missing.

First step from the service manual

Check L1(R)-L2(S), L1(R)-L3(T), and L2(S)-L3(T) phase voltages; if a terminal lacks normal voltage, check external power supply.

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

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What P4 means on a Samsung air conditioner

The P4 error code on a Samsung commercial or multi-zone air conditioner indicates a reverse phase or open phase fault on the incoming 3-phase power supply. The unit's internal phase detection circuit continuously monitors the three input lines, designated L1(R), L2(S), and L3(T), to confirm that all three phases are present and arrive in the correct rotational sequence. When the monitoring circuit detects that the phase order is reversed or that one or more phases are absent entirely, it triggers the P4 fault and shuts down the compressor to prevent mechanical damage.

The most immediate diagnostic step is to measure voltage across all three phase pairs: L1(R) to L2(S), L1(R) to L3(T), and L2(S) to L3(T). Each pair should show balanced line-to-line voltage consistent with the local supply standard (typically 208V, 220V, or 380V depending on the installation). A reading of zero or significantly reduced voltage on any pair points to an open phase condition originating in the external power supply, breaker panel, disconnect switch, or supply wiring. If all three voltages are present but the fault persists, the phase sequence itself may be reversed, which commonly occurs after utility work, a wiring error during installation, or reconnection following service. In that case, swapping any two of the three supply conductors at the disconnect will correct the rotation sequence.

Source: manufacturer service documentation.

Common causes of P4

  1. 01

    Open phase in external power supply

    Wiring / connection

    One of the three supply lines (L1, L2, or L3) has lost voltage due to a blown fuse, tripped breaker pole, or a failed connection at the disconnect switch or panel. Measure voltage across all three phase pairs to identify which leg is missing.

  2. 02

    Reversed phase sequence

    Wiring / connection

    The three supply conductors are connected in the wrong rotational order, often after utility maintenance, a panel upgrade, or a wiring error during installation. Swapping any two of the three incoming line conductors at the disconnect will reverse the sequence.

  3. 03

    Loose or corroded supply wiring termination

    Wiring / connection

    A high-resistance or intermittent connection at a terminal block, disconnect lug, or breaker can cause a phase to drop under load. Inspect all supply-side terminals for signs of burning, corrosion, or inadequate torque.

  4. 04

    Utility supply interruption or imbalance

    Wiring / connection

    A fault on the utility grid can drop or reduce one phase on the incoming service. If other 3-phase equipment at the same location is also faulting, contact the utility company to check the incoming service.

  5. 05

    Failed phase detection board inside the unit

    Part replacement

    If all three supply voltages measure correctly and the phase sequence is confirmed proper but the P4 fault persists, the internal phase detection or control board may be faulty. This is the least likely cause and should only be considered after all external supply conditions are ruled out.

See the test procedure for each cause

Frequently asked questions about P4

What does P4 mean on a Samsung air conditioner?
P4 indicates a reverse phase or open phase error on the 3-phase electrical supply. The unit has detected that the three power lines are either connected in the wrong sequence or that one or more phases are missing entirely. The system shuts down automatically to protect the compressor from running on an incorrect or incomplete power supply.
Can I fix a P4 error myself?
Diagnosing and correcting a P4 error involves working directly with 3-phase line voltage, which is hazardous and should only be performed by a licensed electrician or qualified HVAC technician. The first step is measuring voltage across all three phase pairs at the unit's terminal block. If a phase is missing, the fault lies upstream in the breaker panel or disconnect. If the phase sequence is reversed, swapping two supply conductors at the disconnect will resolve it.
Is it safe to keep running the unit when P4 is displayed?
No. The unit will not operate while P4 is active because it has already shut down to protect the compressor. Attempting to override or bypass the fault protection is not recommended. Running a 3-phase compressor on an open phase or in reverse rotation can cause severe mechanical damage and creates a fire or electrical hazard.
Will resetting the unit clear the P4 error?
A power reset will temporarily clear the fault display, but the P4 code will return immediately when the unit restarts if the underlying supply problem has not been corrected. The phase detection circuit checks supply conditions on every startup. The fault will only stay cleared after the phase sequence is confirmed correct and all three phases show proper voltage.
How much does it cost to repair a P4 error on a Samsung air conditioner?
Cost depends entirely on the root cause. If the issue is a tripped breaker, a loose terminal, or a reversed phase connection, an electrician service call typically ranges from $100 to $300. If the utility supply is at fault, there may be no charge from the utility. In the rare case that the internal phase detection or control board has failed, parts and labor can range from $300 to $800 or more depending on the unit model and board availability.

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