Indoor / Outdoor Communication Error
In short
Mitsubishi E6 means the indoor unit isn't receiving a proper signal from the outdoor unit — a communication or detection error across the S1/S2/S3 connection. It's commonly a wiring fault (loose, mis-landed, or damaged S1/S2/S3 wire), incorrect voltage, or a failed indoor or outdoor control board. The system will fault out and stop conditioning until the link is restored.
Is it safe to keep operating with E6?
No safety hazard from the code itself, but the system stops heating or cooling while E6 is active. Note that on Mitsubishi mini-splits the S1/S2/S3 line carries both power and signal, so a wiring fault here can involve live voltage — always shut off at the breaker before opening terminal covers.
Symptoms
- E6 on the indoor display, remote, or via the blinking operation LED
- System starts then stops after a short run
- No heating or cooling; outdoor unit may not energize
- May appear after an install, a board swap, or a power event
Common causes (most → least likely)
How to diagnose it (before buying parts)
- 1 Switch OFF at the breaker before opening any terminal cover — the S1/S2/S3 line is live.
- 2 Confirm S1/S2/S3 are landed on matching terminals at both indoor and outdoor units and fully tight; re-land any loose or back-stabbed strand.
- 3 Inspect the interconnecting cable end-to-end for pinches, nicks and rodent damage.
- 4 With power on, confirm correct supply voltage at the outdoor unit. Mitsubishi expects defined AC voltages between S1-S2 and a signal between S2-S3 — out-of-range readings localize the fault.
- 5 Measure between terminals per the service manual: a missing S2-S3 signal with correct S1-S2 power points to a board or wire fault rather than supply.
- 6 If wiring and voltages are correct, replace the indoor control board first, then the outdoor board.
Repair options & cost
By model / series
S1/S2/S3 carries power and signal on one cable. Most E6 faults here are a wiring/terminal issue on that 3-wire run — verify it before replacing parts.
E6 can reference a specific indoor unit address. Check the wiring and address of the called-out indoor unit; a single bad branch flags the system.
Uses a transmission (M-NET) bus; verify polarity, bus wiring and termination per the install manual rather than treating it as a simple 3-wire run.
Frequently asked questions
What does E6 mean on a Mitsubishi mini split?
E6 is an indoor-to-outdoor communication / signal-detection error. The indoor unit isn't getting a valid signal from the outdoor unit over the S1/S2/S3 connection, so the system stops running.
What's the most common cause of E6?
Wiring — a loose, corroded, mis-landed or damaged S1/S2/S3 conductor between the units. After that, incorrect outdoor voltage, then a failed indoor or outdoor control board.
Can I fix Mitsubishi E6 myself?
Checking and re-landing the S1/S2/S3 terminals (with power off at the breaker) is a reasonable DIY step. Confirming voltages and replacing a control board is usually best left to an HVAC tech, since the line carries live voltage.
Does E6 mean my compressor is bad?
No. E6 is about communication between units, not the compressor itself. Fix the signal/wiring fault first; the compressor is rarely the cause of an E6.