This site is not affiliated with Daikin; this is an independent explanation.
The Daikin U4 error code is a communication alert. A split or multi split Daikin AC has an indoor unit and an outdoor unit that constantly exchange small signals. On most models U4 appears when that conversation is lost or garbled, so the system stops instead of running blind.
It is often less scary than a compressor code. Sometimes a power dip or a tripped outdoor switch is all that happened. Other times a loose or damaged connecting cable, a water or insect problem on a board, or a failed part sits behind it. Tables are not identical across models, and VRV systems follow their own, so verify with your manual or Daikin authorised service before acting.
Common causes
1Voltage dip or power cut mid cycle
How to tellU4 appeared after a thunderstorm, load shedding or a tripped MCB
What fixes itPower off for a few minutes, then restore and test once
2Outdoor isolator or MCB switched off
How to tellIndoor panel lights up but you hear nothing from outside
What fixes itFind the isolator box near the unit and the breaker at the board; turn on once
3Loose or damaged connecting cable
How to tellCode appears at random, often after shifting furniture or repair work
What fixes itTechnician inspects the inter unit wiring and terminals
4Moisture, insects or rodent damage
How to tellRainy season trouble, signs of chewed wire or nests near the unit
What fixes itTechnician checks and cleans the boards and cabling
5Faulty indoor or outdoor control board
How to tellU4 stays after the wiring check and a clean restart
What fixes itAuthorised service tests and replaces the faulty board
6Unstable supply at the building
How to tellOther appliances also dim or reset, lights flicker
What fixes itElectrician checks the supply and earthing; add a suitable stabiliser
How to fix it, step by step
Check whether the outdoor unit has power
Look for the outdoor switch or isolator box near the unit and the AC's own MCB at the main board. Make sure both are on. If one has tripped, switch it back once. If it trips again, do not keep trying.
Do a full power reset
Flick the AC's breaker down after pressing off on the remote, and leave everything alone while the boards settle. Ten quiet minutes is enough. Flick it up, wait a minute, then call for cooling again. A passing glitch usually disappears.
Jot down the code and when it first showed
Write down if it came after rain, a power cut or a move of the outdoor unit. Technicians use this timing to guess whether it is a surge, water entry or a wiring issue.
Look for obvious damage from outside
Without opening anything, check whether the visible pipe and cable bundle between the units looks cut, chewed or soaked. Keep your hands away from live parts. Report what you see to the technician.
Try the other mode or a different set point
Ask for cooling at a middling temperature. If U4 shows straight away the link is broken. If it takes a while, a supply dip or heat related fault is more likely.
Call authorised service for wiring tests
Checking the inter unit cable, terminals and boards needs a trained person with the right meters. Mains parts are dangerous. Do not open either unit or touch the boards.
What the Daikin U4 error code means in plain words
Imagine a phone call where the line cuts out halfway. The indoor unit sends orders and the outdoor unit reports back. Once the replies stop, the indoor unit raises U4. It cannot say which end failed, only that the link did.
You press the remote, the indoor fan starts, but the outdoor unit stays silent. A little later the display shows U4. That hints that the outdoor side has no power or is not answering, so its switch is the first thing to check.
Daikin VRV and cassette units
On bigger Daikin VRV systems and ceiling cassette units, a U4 style fault can involve many indoor units on one line, so the technician needs the system layout. If several rooms show the alert together, the cause is probably the shared outdoor or the main control wiring, not each indoor unit.
Commercial systems are installed and maintained by trained contractors. Do not reset repeatedly or open any panel. Call the installer or authorised service and share the exact code shown.
Daikin inverter U4 error code reset
A power reset is safe and worth one attempt on any Daikin inverter AC showing this code. If the code returns right away, the fault is real and further resets only waste time and strain the parts. After a reset that works for days, still ask your technician to check the cable terminals at the next service, because loose joints cause repeating faults.
Preventing U4 communication faults
Fit a proper stabiliser or surge protection if your area has swings, keep the outdoor unit away from standing water, seal the pipe entry points against insects, and book yearly service. For a general look at weak cooling, read AC not working, and see the AC error codes hub.
Frequently asked questions
What does U4 mean on a Daikin AC?
It usually flags a lost link between the indoor and outdoor halves of the system. Power trouble, wiring faults and board faults can all trigger it, so match it with your manual.
Is there a quick reset for Daikin U4?
Yes. Stop the AC from the remote and cut the breaker for roughly ten minutes before restoring it. If U4 pops up again, hand it to authorised service.
Is the Daikin inverter AC U4 error serious?
It is usually less costly than a compressor fault, but it can stop the AC completely. The fix may be a simple switch or a wiring or board repair.
Can a power cut cause the Daikin U4 error?
Yes. A cut or voltage dip can drop the link between units. A reset often clears it. A repeating code points to wiring or boards.
Should I open the outdoor unit to check the wires?
No. The outdoor unit has live parts and the boards are sensitive. Only an authorised technician should check the connecting cable and terminals.