U0Daikin · VRV IV RXTQ-TA / RXTQ-TB · Heat Pump

Daikin VRV IV RXTQ-TA / RXTQ-TB error code U0

Refrigerant shortage, warning that should not be ignored

Needs attention soon

Also written as refrigerant leak, loss of charge, low charge, U0

Which models this applies to. System

Quick answer

On Daikin VRV IV RXTQ-TA / RXTQ-TB, U0 is a fault. It means refrigerant shortage, warning that should not be ignored. A first thing to check: A refrigerant leak anywhere in the system.

Applies to: Daikin VRV IV RXTQ-TA / RXTQ-TB; Daikin VRV IV RXTQ heat pump codes · Heat Pump
Type of code: Fault; Urgency: Needs attention soon
First safe check: Look for oil staining or a frost patch along the refrigerant lines, which is where a leak shows first.

Checked 2026-09-26

What U0 means on this model

The system has detected it does not have enough refrigerant. The operation lamp stays lit and the unit keeps running, which is why this code is dangerous: a system short of refrigerant runs a compressor without enough refrigerant to cool its own windings, and continued running destroys the compressor permanently. Treat this as a stop and investigate even though the lamp is on.

Symptoms you will notice

Indoor or outdoor unit shows U0 with the operation lamp lit
The system still produces some heating or cooling but much less than it should
The unit runs continuously and never reaches setpoint
The discharge pipe is unusually hot
Ice or frost on part of the refrigerant line
A pressure cutout or low pressure code appears after a period of running

Common causes

The code tells you what the control detected, not which part failed. These are the causes we see most often, roughly in order of likelihood:

A refrigerant leak anywhere in the system
A leak at a brazed joint, a flare, a valve or a service connection
A leak at the outdoor unit or at an indoor unit coil
An incorrect initial charge at installation
A failed expansion valve that has leaked past its seat

What you can safely check

These are homeowner-level checks. None of them need you to open a live electrical compartment or work on gas.

Look for oil staining or a frost patch along the refrigerant lines, which is where a leak shows first.
Note that the system still making some heat or cold is not evidence the charge is adequate. A short-charged system will run indefinitely without ever reaching temperature.
Check the supply voltage, since a marginal supply and a low charge can produce similar symptoms.

Do not do these

×Do not ignore U0 because the operation lamp stays lit. This is the warning code that most often leads to a dead compressor, because continued running destroys the compressor windings.
×Do not add refrigerant without finding and fixing the leak. Any refrigerant added to a leaking system ends up in the atmosphere and the leak is still there.
×Do not attempt to find a leak by listening for it alone. A small leak can take months to empty a system and is easier to find with a proper leak detector.

When to call a technician

Book a technician, do not keep resetting it

This fault will not clear on its own. Every power cycle gives the control one more failed attempt, and repeated attempts are what turn a cheap part replacement into a seized or contaminated system.

The refrigerant circuit is sealed and legally requires licensed handling. Opening it without certification releases refrigerant and can put the whole system in violation of regulation, so a low-pressure or high-pressure fault on this equipment is a call-out, not a weekend job.
If the appliance has stopped mid-cycle, is making a noise you have not heard before, or smells burnt, stop reading here and call. Those are symptoms of something the fault code does not describe well enough.

A licensed technician job. Noticing oil staining or a frost patch is reasonable for a homeowner.

What it costs to fix

Parts

Refrigerant by the pound $40 to $120. Repair parts vary: a flare fitting $10 to $40, a service valve $20 to $60.

Labor

Two to five hours to find, repair, recover and recharge.

DIY or technician