U0

Daikin Air Conditioner

Severity: Critical

What it means

Daikin air conditioner U0 is the refrigerant-shortage / EEV-failure error documented on Daikin Global's error codes page.
Daikin's published description: 'low pressure drop due to refrigerant shortage or electronic expansion valve (EEV) failure.'
U0 fires when the outdoor unit's low-pressure sensor measures pressure below the operating threshold — usually because refrigerant has slowly leaked over years of operation, but sometimes because the EEV that meters refrigerant flow is stuck closed.
U0 is not a code you can fix without HVAC tools and an EPA Section 608 certification (in the US) or local refrigerant licence elsewhere.

Affected Models

Daikin split-system residential air conditioners (FTX, RX series)
Daikin Sky Air commercial split systems
Daikin VRV systems (U0 on the master outdoor unit)
Daikin heat pump models (where U0 affects both heating and cooling capacity)
U0 is documented across Daikin's split, multi-split, and VRV fault code lists

Common Causes

Refrigerant leak in the line set, indoor coil, or outdoor coil
Refrigerant slowly outgassed through Schrader valve cores
Electronic Expansion Valve (EEV) stuck closed, blocking refrigerant flow
Low-pressure sensor itself faulty (rare — usually after the leak is fixed)
Recent service work left the system undercharged

How to Fix It

Stop running the AC.

Turn the unit off at the remote.
Don't keep cycling it on hoping U0 will clear.
Running a Daikin compressor with low refrigerant overheats it — and a compressor replacement is an expensive sealed-system repair compared to a leak fix.
Leave the unit off until a technician can attend.
Check for visible signs of a leak.

While waiting for service, look at the copper line set between the indoor and outdoor unit.
Oil stains at flare nuts, joints, or along the lineset point to a refrigerant leak (refrigerant oil escapes with the gas).
Note where any oil staining is and tell the technician — it saves them an hour of leak hunting.
Note when U0 first appeared.

If U0 appeared during a hot day after years of normal operation, slow refrigerant loss is the most likely cause.
If U0 appeared right after recent service work (a flare reconnected, a coil replaced), the service work probably left a leak.
If U0 appeared with cooling capacity dropping over weeks, slow leak.
If U0 appeared suddenly with a hissing noise, large leak.
Tell the technician the timing — it changes the leak-hunt approach.
Schedule HVAC service with refrigerant capability.

A general handyman cannot fix U0.
You need a technician licensed to handle refrigerant — EPA Section 608 in the US, F-Gas in Europe, or equivalent local certification.
Daikin-trained technicians are preferable because they have access to the model-specific charge weights.
Schedule through daikin.com or your installer's service department.
Expect leak repair + recharge, not just a top-up.

Any HVAC technician topping up refrigerant without finding and fixing the leak is doing a temporary patch.
The refrigerant will leak out again in months and U0 will return.
A proper U0 fix is: pressure-test the system, find the leak with an electronic detector or UV dye, repair the leak (re-flare, re-braze, or replace the leaking section), pull a vacuum to evacuate moisture, recharge by weight to Daikin's spec.
This is a several-hour visit, not a quick top-up.
Consider the EEV path if no leak is found.

If the technician pressure-tests and the system holds pressure (no leak), the EEV is the suspect.
The EEV is an electronically-controlled valve that meters refrigerant into the indoor coil — when it's stuck closed, the low-pressure sensor sees the same shortage symptom as a leak.
EEV replacement is a sealed-system repair like a compressor change — it's expensive but rarely needed compared to leak fixes.

When to Call a Professional

U0 always needs an HVAC technician.
The homeowner cannot legally or safely top up refrigerant in most countries.
The fix requires leak detection (electronic detector or UV dye), repair of the leak, evacuation of the system, and recharge to the manufacturer's specified weight — none of which is owner-doable.
Continuing to run an AC with U0 will damage the compressor.

Frequently Asked Questions

Can I add refrigerant myself with a recharge kit from the hardware store?

No — and not for safety reasons alone.
In most countries (US, EU, UK, Australia, Canada, Israel, etc.) it's illegal to handle HFC refrigerants without certification.
Beyond the legal issue, the 'recharge kits' sold at hardware stores are made for car AC (R-134a) and are completely incompatible with Daikin home AC refrigerants (R-32, R-410A) — connecting one to a Daikin would destroy fittings and inject the wrong substance.
Even if you had the right refrigerant and tools, charging by gauge instead of by weight gets the charge wrong by 20-40% which damages efficiency and reliability.
U0 is a 'call a tech' code, every time.