Daikin VRV IV RXTQ-TA / RXTQ-TB error code U9
Transmission error within the same system
Which models this applies to. System
Quick answer
On Daikin VRV IV RXTQ-TA / RXTQ-TB, U9 is a fault. It means transmission error within the same system. A first thing to check: An indoor unit that has dropped off the communication bus.
- 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 which indoor units are offline, because the offline group points at a specific point in the bus rather than the whole run.
What U9 means on this model
Indoor units that share a refrigerant system cannot see each other over the bus, and the system has shut down. Part of the system has dropped off the bus, so identifying which indoor units are offline points directly at where the break is.
Symptoms you will notice
- Outdoor unit shows U9 and the system stops
- Some indoor units work while others are offline
- A newly added indoor unit is not communicating
- The fault appeared after field settings or addressing were changed
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:
- An indoor unit that has dropped off the communication bus
- A loose or disconnected communication connection at an indoor unit
- An addressing or field settings error after a change
- A failed indoor unit PCB
- An indoor unit that has been removed or swapped without reconfiguring the system
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 which indoor units are offline, because the offline group points at a specific point in the bus rather than the whole run.
- Check the communication connection at the last indoor unit before the offline group, since that is where the break usually is.
- Check that the field settings and addresses still match the equipment actually installed.
Do not do these
- ×Do not re-address the whole system to clear U9 without knowing which unit dropped off. Changing addresses makes the fault harder to find.
- ×Do not work on the communication wiring with the power switch on.
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
Indoor unit PCB $200 to $450. Communication cable $30 to $150.
Labor
Two to four hours to trace the bus and correct it.
DIY or technician
A technician job.
Typical US part and labor ranges, 2026. They vary by region, model and contractor, so treat them as a planning figure, not a quote.
Scope of this code
- Code table
- Daikin VRV IV RXTQ heat pump codes
- Models in scope
- Daikin VRV IV RXTQ-TA / RXTQ-TB
- Checked
- Confirm on your own unit
- Find the model number on the data plate, then check this code in the service manual for that model. Manufacturers revise code tables between model years, and a revised board can renumber a fault.