Code: U3🔧 Daikin Mini Split

Daikin Error Code U3: Check Operation Not Executed (Meaning, Causes, Fixes)

Reviewed 2026-10-24 · Based on 5 manufacturer & service sources

What the code means

Daikin's service manuals call U3 "Check Operation not Executed" and apply it to all outdoor unit models in the SkyAir (SiUS25-501) and VRV manuals. The unit checks whether the check operation has been carried out, and the malfunction is decided when the unit starts operation without it. Daikin's SkyAir manual says the check operation is mandatory for normal unit operation and that U3 is displayed when it hasn't been run. During the check operation, which takes about 15 minutes and up to about 30, the unit checks for wrong wiring, checks that the stop valves are open, and judges the pipe length automatically. Daikin's simple self-diagnosis chart lists U3 as "Check operation not executed or transmission error". On newer VRV systems (RXYQ-XA), U3 carries a sub code that points to why the test run is missing or ended early.

Likely causes

Safe DIY checks

⚠ Before you start

Turn off the power to the outdoor unit and all indoor units before any inspection. Daikin's manuals warn to turn off the power switch before connecting or disconnecting connectors, or parts may be damaged. The outdoor board shares the electrical box with inverter capacitors that can hold high voltage after power is off, and the SkyAir manual says to turn the power on 6 hours before starting operation to protect the compressor, so leave the check operation to the installer or an HVAC technician.

  1. Think about what just happened. U3 nearly always shows on a new installation or after a board replacement, so call the installer or the technician who did the work first.
  2. Write down the full code, including any sub code after U3 (for example U3-03 or U3-07) on newer VRV controllers. It tells the technician which step to look at.
  3. Don't keep restarting the system expecting U3 to clear. Daikin's manual decides the error every time the unit starts without a completed check operation.
  4. Don't open the outdoor unit to press the TEST (BS4) button yourself. The check operation is part of commissioning, and the board sits next to high-voltage inverter parts.

🛠 When to call a pro

  • The check operation hasn't been run. Daikin's flowchart has the technician press and hold the TEST button (BS4) on the master outdoor unit board for 5 seconds or more to start it, with the outdoor unit's front panel mounted so it doesn't misjudge.
  • During the check operation, the SkyAir manual says H2P flickers on the outdoor board and the remote controller shows Test Operation. H3P on means normal completion; H2P and H3P on means abnormal completion, and the technician reads the remote controller's code and corrects it.
  • On RXYQ-XA VRV systems, the technician follows the sub code: U3-03 means run the test operation, U3-04 means check the refrigerant piping and then run it, U3-05 and U3-06 mean run it again because it ended early, and U3-07 and U3-08 mean check the indoor-outdoor transmission and then run it.
  • The check operation was completed and U3 still shows. Daikin's flowchart replaces the main PC board on the outdoor unit.

Parts that commonly fix it

Always match the part number to your exact model.

⚙

Outdoor unit main control board

Daikin's flowchart replaces it only when the check operation was completed and U3 still shows; match the exact outdoor model number.

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⚙

Multimeter

Used by the technician to check power and indoor-outdoor transmission wiring when the test run fails on a wiring sub code.

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Sources

  1. Daikin SkyAir Inverter service manual SiUS25-501, 4.36 "U3" Outdoor Unit: Check Operation not Executed (detection, supposed cause, BS4 flowchart), Daikin Comfort
  2. Daikin SkyAir Inverter service manual SiUS25-501, 1.1.3 Check Operation (mandatory, U3 if not executed, 15 to 30 minute checks, H2P/H3P LEDs), Daikin Comfort
  3. Daikin RXYQ-XA service manual SiUS342003E, 3.61 Check Operation not Executed (U3 sub codes U3-03 to U3-08), via ManualsLib
  4. Daikin VRV II RXYQ8~30MY1K service manual Si39-502A, 3.42 "U3" Outdoor Unit: Check Operation not Executed, via ManualsLib
  5. Daikin Industries, Simple Self-Diagnosis by Malfunction Code (U3 check operation not executed or transmission error)

Last reviewed 2026-10-24. Always confirm against the manual for your exact model number.