Daikin VRV IV RXTQ-TA / RXTQ-TB error code M8
Transmission error between centralized controllers
Which models this applies to. System
Quick answer
On Daikin VRV IV RXTQ-TA / RXTQ-TB, M8 is a fault. It means transmission error between centralized controllers. A first thing to check: A communication fault between centralized controllers.
- 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 addressing of each controller, because a duplicate or wrong address breaks the bus even when the wiring is right.
What M8 means on this model
Two or more centralized controllers cannot communicate with each other over the bus that shares information between them. Heating and cooling continues, but the system loses coordinated control and reporting.
Symptoms you will notice
- System shows M8
- Controllers show different information from each other
- One controller or group of zones is not appearing on the others
- The fault followed adding a controller or a building network change
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 fault between centralized controllers
- A loose or disconnected bus connection between controllers
- An addressing error after a controller was added
- A failed controller board
- A network configuration change that broke 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 addressing of each controller, because a duplicate or wrong address breaks the bus even when the wiring is right.
- Check the bus connections between controllers, particularly at the ones most recently added.
- Check whether a building network or controller change happened before the fault appeared.
Do not do these
- ×Do not assume the heating and cooling has failed. M8 is a controller network fault and the zones usually still run.
- ×Do not add controllers to the bus without checking the address plan, because a collision produces this same code.
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 building system.
What it costs to fix
Parts
Centralized controller $400 to $1,200. Bus cable $40 to $150.
Labor
Two to four hours to correct the bus or the addressing.
DIY or technician
A technician job for a multi-controller building 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.