How House Works

Daikin U4: indoor-outdoor communication failure

By How House Works EditorialBased on Daikin manualsChecked against sources Updated

Daikin remote controller display

U4

“FAILURE OF TRANSMISSION (BETWEEN INDOOR UNIT AND OUTDOOR UNIT)”

FTXM-WVJU9, FTXQ-ASBU9, FTX/FTXN-NMVJU, FTXS30/36 · FTXM-WVJU9 Operation Manual, p. 50; FTXQ-ASBU9 Operation Manual, p. 28; FTX/FTXN-NMVJU Operation Manual, p. 30; FTXS30/36 Operation Manual, p. 36
Technician

In the SYSTEM group. The FTXM-W/V service manual calls U4 “Signal transmission error (between indoor unit and outdoor unit)” (FTXM-W/V Service Manual SiUS042113EA, p. 96).

What it means

The service manual describes a check of the signal transmission data received from the outdoor unit: U4 appears when that data cannot be received normally or its content is abnormal FTXM-W/V Service Manual SiUS042113EA, p. 108. The FTK-B/FTX-B and FTX-W service manuals describe the same check FTK-B/FTX-B Service Manual DAMA-SM-23-006, p. 65; FTX-W Service Manual SiUS042228E, p. 78.

The owner’s tables put U4 in the SYSTEM group, not under the indoor or the outdoor unit FTXM-WVJU9 Operation Manual, p. 50; FTXS30/36 Operation Manual, p. 36. Among the causes the service manual lists a break in connection wire No. 3 between the two units, so a U4 does not by itself say which unit is at fault FTXM-W/V Service Manual SiUS042113EA, p. 108.

Check these first

The owner’s manuals have no step written for U4 itself. What they give for any blinking lamp:

  1. Check that the OPERATION lamp blinks at an interval of about 0.5 seconds. That is the unit’s notice of an abnormality FTXM-WVJU9 Operation Manual, p. 49; FTXQ-ASBU9 Operation Manual, p. 27.
  2. Read the code on the remote, as described in the Daikin code guide, and write it down.
  3. Turn off the power with the circuit breaker, then restart with the remote controller FTXM-WVJU9 Operation Manual, p. 46; FTXQ-ASBU9 Operation Manual, p. 24; FTX/FTXN-NMVJU Operation Manual, p. 27. If you stopped the unit yourself, the manual’s caution is to wait at least 5 minutes after stopping before turning off the power, or water may leak FTXM-WVJU9 Operation Manual, p. 4. Right after the breaker goes on, the unit takes about 3 to 10 minutes to prepare before it runs FTXM-WVJU9 Operation Manual, p. 47; FTXQ-ASBU9 Operation Manual, p. 25.
  4. Watch the lamp. In the FTXM-WVJU9 and FTXQ-ASBU9 charts U4 is one of the “Other error codes”: if the lamp stays lit after the unit has run for a while, keep using it; if it blinks again, turn off the circuit breaker and call your dealer FTXM-WVJU9 Operation Manual, pp. 49–50; FTXQ-ASBU9 Operation Manual, pp. 27–28.

On FTXS30/36 units the manual’s steps for a flashing lamp are to clean the air filters, take away anything blocking the indoor or outdoor unit, turn the breaker off and on again, and call the service shop if the lamp still flashes FTXS30/36 Operation Manual, p. 34. The FTX/FTK-BXVJU poster says to call your local dealer or serviceman if the fault persists FTX/FTK-BXVJU Operation Manual, p. 1, section 10.

When to call a licensed technician

Past the restart, U4 is service work. The FTXM-W/V service manual lists these causes FTXM-W/V Service Manual SiUS042113EA, p. 108:

  • supply voltage out of specification, a drop in supply voltage, or a disturbed power supply waveform;
  • a wiring error, or a break in connection wire No. 3 between the indoor and outdoor units;
  • a defective outdoor or indoor unit PCB;
  • a short circuit inside the fan motor winding.

The FTK-B/FTX-B and FTX-W manuals give the same list FTK-B/FTX-B Service Manual DAMA-SM-23-006, p. 65; FTX-W Service Manual SiUS042228E, p. 78. The AXVJU manual names faulty PCBs, a wiring error, a disturbed power supply waveform and broken connection wires AXVJU Service Manual SM-5WM-Y-NA-B1, p. 59. The FTXS-L manual leaves out the supply voltage and fan motor items FTXS-L Service Manual SiUS091133, p. 109.

These checks are on the power supply and the wiring between the units. The owner’s manual says not to do electrical work unless you are licensed to do so FTXM-WVJU9 Operation Manual, p. 4. The service manual warns that working on equipment connected to power may cause an electric shock, and that the outdoor unit’s step-up capacitor must be discharged before repair work FTXM-W/V Service Manual SiUS042113EA, pp. 2–3.

Notes for different models

  • FTX/FTK-BXVJU and the BRC52B and BRC51D61 remote manuals call U4 “Communication error” FTX/FTK-BXVJU Operation Manual, p. 1, section 9; BRC52B Remote Controller Manual, p. 1-4; BRC51D61 Wired Remote Controller Manual, p. 12. The same lists use U7 and LC for communication errors between the outdoor control PCB and the IPM or inverter PCB.
  • The FTX-W and FTXS-L service manuals call it “Signal transmission error (between indoor unit and outdoor unit)” FTX-W Service Manual SiUS042228E, p. 68; FTXS-L Service Manual SiUS091133, p. 94. The AXVJU manual calls it “COMMUNICATION ABNORMALITY” AXVJU Service Manual SM-5WM-Y-NA-B1, p. 34.
  • Subcodes such as U4-01 do not appear in any Daikin wall-unit manual used here.

Related Daikin codes

  • UA “INDOOR-OUTDOOR UNIT COMBINATION FAULT”
  • U2 “DROP VOLTAGE OR MAIN CIRCUIT OVERVOLTAGE”
  • U7 “Communication error (outdoor control PCB and IPM PCB)”

See also

Other ductless mini-splits: Fujitsu, Mitsubishi Electric, MrCool

All Daikin codes

Daikin is a trademark of its owner. How House Works is not affiliated with or endorsed by Daikin.

Sources

  1. Daikin Comfort Technologies North America (document library). FTXM-WVJU9 Operation Manual. 3P686871-1A, page stamp 2022/05/30; covers FTXM09/12/18/24WVJU9. Listed under ATMOSPHERA in Daikin's document library.. Accessed Oct 13, 2026.
  2. Daikin Comfort Technologies North America (document library). FTXQ-ASBU9 Operation Manual. 3P741457-13J, English; page stamp 2024/11/19; covers FTXQ09ASBU9 and FTXQ12ASBU9. Listed under OTERRA 115V. The library list shows only the French and Spanish files of this manual; the English file is served at this address.. Accessed Oct 13, 2026.
  3. Daikin Comfort Technologies North America (document library). FTX/FTXN-NMVJU Operation Manual. 3P379751-4C, page stamp 11/6/2015; covers FTX09/12/15/18/24NMVJU and FTXN09/12/18/24NMVJU. Not in Daikin's current library list; the file is still served from daikincomfort.com.. Accessed Oct 13, 2026.
  4. Daikin Comfort Technologies North America (document library). FTXS30/36 Operation Manual. 3P228444-4K, page stamp 3/28/2013; covers FTXS30HVJU, FTXS36HVJU, FTXS30LVJU and FTXS36LVJU. Not in Daikin's current library list; the file is still served from daikincomfort.com.. Accessed Oct 13, 2026.
  5. Daikin Comfort Technologies North America (document library). FTX/FTK-BXVJU Operation Manual. 3P601789-2 E (cover label); no printed date; covers FTX, FTXB and FTK09/12/18/24BXVJU; R410A. A poster with one page per language (English, Spanish, French) and no page numbers: citations give PDF page 1 and the poster section. The same file is also listed under ENTRA.. Accessed Oct 13, 2026.
  6. Daikin Comfort Technologies North America (document library). BRC52B Remote Controller Manual. OM-BRC52B-0617(2)-DAIKIN USA, Part No. R08019046595B; print stamp 10/30/2017; covers remote controllers BRC52B61/62/63/64/65. Does not name the indoor units. The library lists it under ENTRA.. Accessed Oct 13, 2026.
  7. Daikin Comfort Technologies North America (document library). BRC51D61 Wired Remote Controller Manual. R08019049297B; print stamp 7/12/2022. Does not name the indoor units. The library lists it under OTERRA (cooling only) and ENTRA.. Accessed Oct 13, 2026.
  8. Daikin Comfort Technologies North America (document library). FTXM-W/V Service Manual SiUS042113EA. SiUS042113EA, revision of 01/2023; covers FTXM09/12/18/24WVJU9 and FTXM-VVJU with RXM outdoor units. Accessed Oct 13, 2026.
  9. Daikin Comfort Technologies North America (document library). FTK-B/FTX-B Service Manual DAMA-SM-23-006. DAMA-SM-23-006, first edition 04/2023; covers FTK09/12/18/24BXVJU and FTX09/12/18/24BXVJU with RK/RX-BXVJU outdoor units. Accessed Oct 13, 2026.
  10. Daikin Comfort Technologies North America (document library). AXVJU Service Manual SM-5WM-Y-NA-B1. SM-5WM-Y-NA-B1; no printed date (PDF created 2021-09-29); covers FTKB/FTKN/FTXB/FTXN09/12/18/24AXVJU. Listed under ENTRA.. Accessed Oct 13, 2026.
  11. Daikin Comfort Technologies North America (document library). FTX-W Service Manual SiUS042228E. SiUS042228E, first edition 10/2022; covers FTX30/36WVJU9 with RK30/36WMVJU9 or RX30/36WMVJU9. Listed under POLARA.. Accessed Oct 13, 2026.
  12. Daikin Comfort Technologies North America (document library). FTXS-L Service Manual SiUS091133. SiUS091133; no printed date (PDF created 2013-01-11); covers FTXS09–36LVJU and FDXS09/12LVJU. Not in Daikin's current library list; the file is still served from daikincomfort.com.. Accessed Oct 13, 2026.