Daikin VRV IV RXTQ-TA / RXTQ-TB error code U8
Transmission error between main and sub controllers
Which models this applies to. System
Quick answer
On Daikin VRV IV RXTQ-TA / RXTQ-TB, U8 is a fault. It means transmission error between main and sub controllers. A first thing to check: A loose or disconnected controller-to-controller link.
- Applies to
- Daikin VRV IV RXTQ-TA / RXTQ-TB
- Daikin VRV IV RXTQ heat pump codes · Heat Pump
- Type of code
- Fault
- Urgency: Book a technician
- First safe check
- Check the controller chain connections and confirm each controller is wired in the correct order, since a daisy chain is order dependent.
What U8 means on this model
In a system with more than one controller, the link between the main controller and the secondary controllers has failed. The controllers are daisy chained to share zone information, and a break in that chain stops the secondary controllers updating. Heating and cooling usually keeps working.
Symptoms you will notice
- Indoor or system controller shows U8
- Secondary controllers do not update or do not follow the main controller
- One controller in the chain is unresponsive
- Settings made on the main controller do not appear on a secondary controller
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 loose or disconnected controller-to-controller link
- A failed main or secondary controller
- A daisy chain wired incorrectly so a unit is in the wrong position in the run
- A controller installed beyond the permitted number of controllers on the bus
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 the controller chain connections and confirm each controller is wired in the correct order, since a daisy chain is order dependent.
- Confirm the number of controllers on the bus is within what the system allows.
- Check whether one controller was recently added or moved, because that is when this code appears.
Do not do these
- ×Do not add controllers to the bus beyond the system limit. The bus will not carry them and the chain will keep failing.
- ×Do not assume the controllers are interchangeable in a daisy chain without checking the manual, because their position in the run is what makes the chain work.
When to call a technician
Worth scheduling, and worth your own checks first
You can usually keep living with this for a short while, but it will not fix itself. Do the safe checks below first, because a tripped switch or a blocked airflow is a five-minute fix that would otherwise cost you a service call.
- 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 for a multi-controller system.
What it costs to fix
Parts
Remote controller $120 to $300. Controller cable $20 to $80.
Labor
One to two hours to correct the chain.
DIY or technician
A technician job for a multi-controller system.
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.