If your Mitsubishi City Multi system is showing error code 6600, two devices on the system have been given the same address.
This is a configuration fault, not a broken component. And it matters that you know the difference, because 6600 and the more common 6607 look similar on the controller but lead to completely different repairs.
What this means for your building
Every device on a City Multi system — outdoor unit, indoor units, BC controller, remote controllers — carries a unique address on the M-NET control network. That address is how the system knows which unit it is talking to.
When two devices end up sharing one, the system can no longer tell them apart. Messages meant for one may land on the other. The system flags 6600 and the affected zones start behaving unpredictably: a unit responds to a command intended for a different area, or stops responding at all.
Nothing is breaking. The system simply cannot resolve who is who.
How urgent is it?
Less urgent than most codes on this list. No refrigerant is escaping, no compressor protection is bypassed, and nothing is being mechanically damaged.
What you have is a system you cannot rely on. Zones may condition when they should not, which wastes energy and generates complaints. It should be corrected, but it rarely needs an after-hours call.
What it typically costs
Low — frequently the least expensive fault a VRF system will throw at you.
There is often no part to replace. The cost is the technician time to audit addressing across the system and resolve the conflict. On a large building with many indoor units that audit takes a while, but it is methodical work rather than expensive parts.
What causes it
Almost always a change to the system:
- Indoor units added, replaced or relocated during a tenant improvement, without addressing being updated
- A control board replaced and the addressing not carried across from the old one
- Two units commissioned in parallel by different people who both assigned the same address
- Expansion of the system where a new branch overlapped an existing address range
- A restored or reset device that reverted to a default address already in use
Notice the pattern: 6600 usually follows human intervention. If nothing about your system has changed recently, 6600 is less likely than 6607.
What happens if you leave it
The immediate cost is unpredictable comfort, which in a commercial building is a steady stream of tenant complaints that nobody can reproduce reliably.
The longer cost is subtler. Every future service visit now starts from a system whose configuration cannot be trusted. A technician diagnosing a completely unrelated fault has to first work out whether what they are seeing is the actual problem or an addressing artefact. That adds time to repairs that had nothing to do with 6600.
What we check
- Audit addressing across the whole system — outdoor unit, indoor units, BC controller, and remote controllers.
- Identify the conflicting pair and determine which one is incorrectly assigned.
- Establish what changed — recent equipment work is nearly always the origin.
- Reassign and verify so the corrected address does not collide with anything else.
- Confirm the whole network responds correctly after the change.
- Document the addressing scheme so the next contractor to touch the system does not recreate the problem.
What you can tell us before we arrive
- Has any equipment changed recently? Units added, moved, replaced, or a board swapped. This is the single most useful thing you can tell us.
- Which zones are behaving oddly? Particularly if one area responds to another area’s thermostat.
- Who did the last work on the system? Not to assign blame — it tells us what addressing convention may have been used.
Related codes
- 6607 — No ACK error. A device sent a message and got no acknowledgment. Points to wiring, terminals or power rather than configuration.
- 6602 — Transmission processor hardware error. Indicates a hardware fault rather than either wiring or addressing.
Get it corrected
We are certified on Mitsubishi, Daikin and LG VRF platforms, and we service systems we did not install — including buildings where previous contractors left the addressing in a state nobody documented.
Next business day service for commercial VRF.
Related codes
Error 6607 means part of your VRF control network stopped responding. What it means for your building, how urgent it is, and what it typically costs to fix.
Error 6602 is a transmission processor hardware fault. Unlike other M-NET codes, this one usually points at a board rather than wiring or configuration.
Frequently asked questions
What does error code 6600 mean on a Mitsubishi City Multi system?
Two devices on the M-NET control network have been assigned the same address. The system cannot tell them apart, so messages intended for one may reach the other. It is a configuration problem rather than a hardware failure or a wiring break.
Is 6600 caused by faulty wiring?
Usually not, and this is the key difference from 6607. A 6600 means the addressing is wrong, not that a cable is damaged. Sending a technician to trace control wiring on a 6600 generally wastes a visit, because the fault is in configuration.
Why did 6600 appear after we had work done?
That is the most common trigger. Address conflicts typically appear after indoor units are added, replaced, or relocated during a tenant improvement, or after a board is swapped and the addressing is not carried across. The timing of the change is the strongest clue.
How expensive is a 6600 error to fix?
It is usually one of the cheapest VRF faults. There is often no part to replace at all. The cost is the technician time needed to audit the addressing across the system and correct the conflict, which on a large building is mostly a methodical process.
Can a 6600 error damage my equipment?
Not directly. No compressor or refrigerant protection is being bypassed. The practical risk is operational rather than mechanical: zones may heat or cool when they should not, which wastes energy and generates tenant complaints until it is corrected.
What is the difference between 6600 and 6607?
Both are M-NET faults but they have opposite causes. 6600 means two devices share an address, which is a configuration issue. 6607 means a device sent a message and got no reply, which usually points to wiring, terminals or power. They need different repairs.