Mitsubishi Mini Split Error Code E6: Indoor and Outdoor Units Can't Communicate (Meaning, Causes, Fixes)
What the code means
Mitsubishi Electric lists E6 as "Indoor/outdoor unit communication error (Signal receiving error)" on the wired remote controller check-code tables in its Mr. Slim service manuals (for example the SEZ-KD ducted and PKA-A wall-mounted indoor units). The indoor controller board flags E6 when it cannot receive any normal signal from the outdoor unit for six minutes after power is turned on, or for three minutes during operation. On systems with two or more indoor units on one outdoor unit, it also trips when an indoor board goes three minutes without the outdoor board's signal that allows it to transmit. Its partner code E7 is the transmitting error (the indoor board sends a signal but keeps reading the wrong reply), and E9 is a communication error detected by the outdoor unit. Mitsubishi Electric's Mr. Slim error code chart also shows E6/E7 on the outdoor board's LEDs as a two-blink communication fault, with the same checks: correct indoor/outdoor connecting wire, electrical noise on that wire or the power supply, then a power reset.
Likely causes
- A loose, disconnected or damaged indoor/outdoor connecting wire at the indoor unit terminal block or the outdoor unit, which is Mitsubishi's first check.
- Miswiring of that connecting cable, such as reversed (converse) wiring or a short circuit, often after a new install or a repair.
- Electrical noise getting into the indoor/outdoor connecting wire or the power supply, which corrupts the signal between the two boards.
- A faulty transmitting or receiving circuit on the indoor controller board.
- A faulty transmitting or receiving circuit on the outdoor unit's control circuit board.
- On twin or triple indoor unit systems, a fault on another indoor unit's board or wiring, since all units share the communication line.
Safe DIY checks
Turn off the breaker (or breakers) for both the indoor and outdoor units before any inspection. The cable between Mitsubishi indoor and outdoor units carries line voltage along with the communication signal, so leave terminal blocks and circuit boards to a licensed HVAC technician.
- Write down the exact code and whether it shows E6, E7 or E9. They point to different sides of the same communication line.
- Do a full power reset: switch off the breaker (or breakers) feeding both the indoor and outdoor units, wait a few minutes, then switch them back on. Mitsubishi's manuals say to turn the power off and on again to check whether the error returns. Some systems power the indoor unit from the outdoor unit and some have separate breakers, so turn off every breaker for the system.
- After the reset, give it at least six minutes. E6 can take that long to reappear because of how the error is timed after power up.
- Look outside for obvious damage to the cable that runs between the units, such as chewed wiring, a crushed line set cover or recent landscaping or construction work near the line.
- Think about what changed. E6 that starts right after an installation, a board replacement or electrical work often means a wiring connection problem, so call the installer back.
- Don't open the outdoor unit or the indoor terminal cover yourself. The connecting cable between Mitsubishi units carries line voltage as well as the signal.
🛠 When to call a pro
- E6 comes back after a full power reset. A technician checks the indoor/outdoor connecting wire at both ends for looseness, damage or reversed wiring.
- The wiring checks out. Mitsubishi's next step is reading the LED display on the outdoor control board (or connecting their A-control service tool), then replacing the indoor controller board or the outdoor control circuit board.
- You have two or three indoor heads on one outdoor unit. Mitsubishi notes that every unit on the line needs to be checked, and a different indoor unit's board may be the one at fault.
- The code started after a storm, a power surge or new electrical equipment nearby, since electrical noise on the communication line is a listed cause.
- The system is new or under warranty. Contact the installing contractor or a Mitsubishi Electric Diamond Contractor first.
Parts that commonly fix it
Always match the part number to your exact model.
Indoor/outdoor connecting cable
A damaged or miswired interconnect cable is Mitsubishi's first suspect for E6; a technician should match the wire type to the install manual.
Search eBay →Indoor unit control board
Replaced only after wiring and the outdoor board have been checked; match the exact indoor model number.
Search eBay →Multimeter
Used by the technician to check continuity of the connecting wire and voltages at the terminal blocks.
Search eBay →Sources
- Mitsubishi Electric SEZ-KD Mr. Slim service manual, check code table (E6: indoor/outdoor unit communication error, signal receiving error; 6 minutes after power on or 3 minutes in operation; causes and remedies; E7 transmitting error), via ManyManuals
- Mitsubishi Electric Mr. Slim PKA-A24KA service manual, p.13 (wired remote check codes: E6, E7 indoor/outdoor unit communication error; E9 outdoor unit communication error), via ManualsLib
- Mitsubishi Electric Singapore, Mr. Slim error code chart (indoor E6, E7 indoor/outdoor unit communication error; outdoor LED E6/E7 checks: connecting wire, noise, power reset)
- Mitsubishi Electric EHSC series service manual (OCH503), p.40 (E6 reception error, check connections, power both units off and on, then replace boards), via ManualShelf
Last reviewed 2026-10-24. Always confirm against the manual for your exact model number.