U4Daikin · VRV IV RXTQ-TA / RXTQ-TB · Heat Pump

Daikin VRV IV RXTQ-TA / RXTQ-TB error code U4

Transmission error between indoor and outdoor units

Needs attention soon

Also written as bus communication, indoor outdoor communication, U4

Which models this applies to. System

Quick answer

On Daikin VRV IV RXTQ-TA / RXTQ-TB, U4 is a fault. It means transmission error between indoor and outdoor units. A first thing to check: A communication wiring fault between the indoor and outdoor units.

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: Check that the indoor units in the system are actually present and connected, since an indoor unit that has been removed or left disconnected will stop the whole system from talking.

Checked 2026-09-26

What U4 means on this model

The communication bus between the indoor units and the outdoor unit has failed and the system has shut down. It is a wiring, addressing or board level fault rather than a refrigerant fault, and it is the most common system level code because the bus is sensitive to installation quality and terminations.

Symptoms you will notice

Outdoor unit shows U4 and the system stops
The system will not respond to the controller
All indoor units go off together
The fault appeared after a cable was rerouted or an indoor unit was added

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 communication wiring fault between the indoor and outdoor units
A loose, corroded or badly terminated communication connection
A failed indoor unit PCB or outdoor main PCB
A wiring polarity or daisy chain error at installation
An addressing or configuration error after a field settings change
Communication cable that has been run alongside or parallel to power cable without separation

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.

Check that the indoor units in the system are actually present and connected, since an indoor unit that has been removed or left disconnected will stop the whole system from talking.
Check the communication connections at the outdoor unit and at the last indoor unit in the run, which are the two ends that get disturbed.
Check that field settings and addresses still match the installed equipment after any recent change.

Do not do these

×Do not swap indoor unit boards to chase U4. The bus is shared, and a board swap on a wiring fault costs a board and fixes nothing.
×Do not work on the communication wiring with the power switch on.
×Do not rerun communication cable tight alongside power cable without separation, which is a frequent cause of intermittent U4.

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 technician job.

What it costs to fix

Parts

Communication cable $30 to $150. Indoor unit PCB $200 to $450. Outdoor main PCB $600 to $1,400.

Labor

Two to four hours to trace the bus and correct the fault.

DIY or technician