Code: E9🔧 Mitsubishi Mini Split

Mitsubishi Mini Split Error Code E9: Outdoor Unit Communication (Transmitting) Error (Meaning, Causes, Fixes)

Reviewed 2026-10-24 · Based on 4 manufacturer & service sources

What the code means

Mitsubishi Electric's outdoor unit service manuals (for example the PUHZ-RP125VHA and PUZ-A18NHA2) list E9, with the four-digit equivalent 6841, as "Indoor/outdoor unit communication error (Transmitting error) (Outdoor unit)." The outdoor controller circuit board flags it in two situations: when it transmits a "1" on the communication line but reads back a "0" 30 times in a row, or when it can't find a free moment on the transmission line to send for 3 minutes. Mitsubishi's City Multi service handbook describes 6841 the same way and calls it a control communication synchronism that does not recover. The neighboring codes help narrow things down: E6 and E7 are the indoor unit's receiving and transmitting errors, and E8 (6840) is the outdoor unit failing to receive anything normally for 3 minutes. E9 specifically means the outdoor side is the one that can't transmit.

Likely causes

Safe DIY checks

⚠ Before you start

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, and the outdoor board sits next to high-voltage inverter parts, so leave terminal blocks and circuit boards to a licensed HVAC technician.

  1. Write down the exact code. E9 points at the outdoor unit's transmitting side, while E6, E7 and E8 point at other parts of the same communication line, and a technician will want to know which one you saw.
  2. Do a full power reset: switch off every breaker feeding the indoor and outdoor units, wait a few minutes, then switch them back on. Mitsubishi's remedy is to turn the power off and on again and see whether the error returns.
  3. After power comes back, give the system several minutes to run. E9 can take up to 3 minutes of failed transmitting before it shows again.
  4. Look outside for obvious damage to the cable or line set cover running between the units, such as chewed wiring, crushed covers or recent digging, trimming or construction nearby.
  5. Think about what changed recently. E9 right after an installation, a repair or new electrical equipment on the same circuit points toward a wiring connection or electrical noise, so call the installer or an electrician first.
  6. Don't open the outdoor unit or the indoor terminal cover yourself. The connecting cable between Mitsubishi units carries line voltage along with the signal.

🛠 When to call a pro

  • E9 comes back after a full power reset. A technician checks the indoor/outdoor connecting wire at both units for disconnection or looseness, which is Mitsubishi's first check.
  • The wiring checks out and E9 keeps returning. Mitsubishi's service manuals say to replace the outdoor controller circuit board when the error is displayed again after a power reset.
  • The code showed up after a storm, a power surge or new equipment added nearby, since noise on the power supply or the connecting wire is a listed cause and may need an electrician to track down.
  • You have more than one indoor unit on the same outdoor unit, since every unit on the communication line needs to be checked.
  • 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 loose or poorly connected interconnect wire is Mitsubishi's first suspect for E9; a technician should match the wire type to the install manual.

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Outdoor unit control board

Mitsubishi's remedy when E9 returns after a power reset; match the exact outdoor model number before ordering.

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Multimeter

Used by the technician to check the connecting wire and voltages at the terminal blocks.

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Sources

  1. Mitsubishi Electric PUHZ-RP125VHA service manual, p.35 (E9 / 6841: indoor/outdoor unit communication error, transmitting error, outdoor unit; detection, causes and remedy), via ManyManuals
  2. Mitsubishi Electric PUZ-A18NHA2 service manual, p.41 (E8 / 6840 and E9 / 6841 outdoor unit communication errors), via ManualsLib
  3. Mitsubishi Electric R410A (City Multi) service handbook, p.231 (error code 6840 and 6841 definitions, causes, check method and remedy), via ManualsLib
  4. 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

Last reviewed 2026-10-24. Always confirm against the manual for your exact model number.