How House Works

Daikin U0: refrigerant shortage, what to do

By How House Works EditorialBased on Daikin manualsChecked against sources Updated

Daikin remote controller display

U0

“REFRIGERANT SHORTAGE”

FTXM-WVJU9, FTXQ-ASBU9, FTX/FTXN-NMVJU, FTXS30/36, FTX30/36WVJU9, FTXS-L · FTXM-WVJU9 owner's p. 50; FTXQ-ASBU9 owner's p. 28; FTX/FTXN-NMVJU owner's p. 30; FTXS30/36 owner's p. 36; FTX-W service p. 68; FTXS-L service p. 94
Technician

In the SYSTEM group. The FTXM-WVJU9 and FTXQ-ASBU9 manuals put U0 in a box that leads to “Turn off the circuit breaker and call your dealer.” (FTXM-WVJU9 Operation Manual, pp. 49–50; FTXQ-ASBU9 Operation Manual, pp. 27–28). The service manual tables mark U0 “Displayed only when system-down occurs” (FTX-W Service Manual SiUS042228E, p. 68; FTXS-L Service Manual SiUS091133, p. 94).

What it means

U0 is the unit’s judgment that the system is short of refrigerant. The FTXM-W/V service manual describes three ways the unit detects it, all from operating data FTXM-W/V Service Manual SiUS042113EA, p. 104:

  • Input current against compressor running frequency. With too little refrigerant the input current tends to be lower than normal; the condition has to last 7 minutes.
  • Discharge pipe temperature against the opening of the electronic expansion valve. With too little refrigerant the discharge pipe temperature tends to rise; this condition has to last 80 seconds.
  • The difference between suction and discharge temperature, with limits of 7°F (4.0°C) for the 09/12 class and 6°F (3.5°C) for the 18/24 class.

If U0 repeats, the system shuts down, and it resets after about 60 minutes of continuous running without any other error. The service table marks U0 “Displayed only when system-down occurs” FTXM-W/V Service Manual SiUS042113EA, pp. 96, 104.

The service manuals list causes besides a leak. The FTXS30/36H manual names a disconnected thermistor (discharge pipe, indoor or outdoor heat exchanger, room or outdoor temperature), a closed stop valve, refrigerant shortage from a leak, poor compression performance of the compressor, and a defective electronic expansion valve FTXS30/36H Service Manual SiUS04-924_A, p. 105. The AXVJU manual gives the same list AXVJU Service Manual SM-5WM-Y-NA-B1, p. 57.

Check these first

There is no refrigerant check for an owner in these manuals; they say never to inspect or service the unit yourself FTXM-WVJU9 Operation Manual, p. 3. What you can do:

  1. Check that the OPERATION lamp blinks at about 0.5-second intervals and read the code, as described in the Daikin code guide FTXM-WVJU9 Operation Manual, p. 49.
  2. On FTXM-WVJU9 and FTXQ-ASBU9 units, follow the manual’s box for this code. It reads “In the case of error code U0, F3 or F8” on FTXM-WVJU9 and “U0 or F3” on FTXQ-ASBU9, and leads to “Turn off the circuit breaker and call your dealer.” FTXM-WVJU9 Operation Manual, pp. 49–50; FTXQ-ASBU9 Operation Manual, pp. 27–28.
  3. On FTX/FTXN-NMVJU and FTXS30/36 units the manuals have only the general rule for a blinking lamp: restart with the circuit breaker, and if the lamp still blinks, check the code and call your dealer or the service shop FTX/FTXN-NMVJU Operation Manual, p. 27; FTXS30/36 Operation Manual, p. 34.
  4. On FTX/FTK-BXVJU units the poster says that if the air conditioner is not cooling properly, the refrigerant may be leaking, so call your dealer FTX/FTK-BXVJU Operation Manual, p. 1, safety precautions.

FTXM-WVJU9 and FTXQ-ASBU9 units are filled with R32, which the FTXQ manual marks as refrigerant safety group A2L FTXQ-ASBU9 Operation Manual, p. 3. For a leak the FTXM manual says to consult your dealer. It warns that refrigerant gas is heavier than air and a massive leak could lead to oxygen depletion, especially in basements; that refrigerant gas may produce toxic gas in contact with fire, such as from a fan heater, stove or cooking device; and that refrigerants may not have an odor FTXM-WVJU9 Operation Manual, p. 3.

When to call a licensed technician

Every U0 cause in the service manuals is technician work: thermistor connectors, the stop valve, the compressor, the expansion valve and the refrigerant circuit. The repairs in the FTXM-W/V flow chart include reconnecting or replacing thermistors, opening the stop valve, repairing a pipe flare or replacing a union, and “Change for a specified amount of fresh refrigerant.” FTXM-W/V Service Manual SiUS042113EA, p. 105.

Refrigerant work is for a technician certified under EPA Section 608. Daikin’s service manual says: “Servicing shall be performed only as recommended by the manufacturer and licensed or certified in their jurisdiction.” FTXM-W/V Service Manual SiUS042113EA, p. 2. The FTX/FTK-BXVJU poster asks that repairs which include adding refrigerant be checked with Daikin’s service staff, and says not to use the unit until a qualified service person confirms the leak has been repaired FTX/FTK-BXVJU Operation Manual, p. 1, safety precautions.

Notes for different models

  • FTX/FTK-BXVJU, the BRC52B and BRC51D61 remote manuals and the AXVJU service manual call U0 “Insufficient gas” FTX/FTK-BXVJU Operation Manual, p. 1, section 9; BRC52B Remote Controller Manual, p. 1-4; BRC51D61 Wired Remote Controller Manual, p. 12; AXVJU Service Manual SM-5WM-Y-NA-B1, p. 34.
  • The FTK-B/FTX-B service manual lists U0 in its code definitions but not in table 4.1, which points to the check pages, so it has no U0 check page FTK-B/FTX-B Service Manual DAMA-SM-23-006, pp. 57–58. The FTX-W manual lists U0 in table 4.1 with no reference page FTX-W Service Manual SiUS042228E, p. 68.
  • On FTXM-WVJU9 the same box covers F8; the FTXQ-ASBU9 box names only U0 and F3 FTXQ-ASBU9 Operation Manual, p. 27.

Related Daikin codes

  • F3 “HIGH TEMPERATURE DISCHARGE PIPE CONTROL”
  • E5 “OL (COMPRESSOR OVERLOAD) STARTED”
  • EA “COOLING-HEATING SWITCHING ERROR”

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.
  13. Daikin Comfort Technologies North America (document library). FTXS30/36H Service Manual SiUS04-924_A. SiUS04-924_A; copyright 2011; covers FTXS30/36HVJU with RKS/RXS30/36HVJU. Listed under SkyAir in Daikin's document library.. Accessed Oct 13, 2026.