Quick answer
Mitsubishi error code E6 is an indoor/outdoor unit communication error: the indoor unit has detected a signal-receiving error on the link to the outdoor unit. There is no heating or cooling until communication is restored. Turn the power off at the breaker and on again once, which is Mitsubishi's own re-check step. If E6 returns, the connecting wire and both controller boards need testing.
If a Mitsubishi controller is showing error code E6, it means the indoor unit cannot hear the outdoor unit. Mitsubishi's self-diagnosis table for the PAR-21MAA wired controller on Mr. Slim systems defines E6 as an indoor/outdoor unit communication error, specifically a signal-receiving error detected by the indoor unit. Check codes are not identical across every Mitsubishi line, so the manual and the label for your model outrank this page.
What the indoor unit is telling you with E6
The detail in the definition is who detected the fault. The indoor unit was listening for the outdoor unit's signal and did not receive it properly. That points at three things: the wire between the units, interference on that wire or on the power supply, and the controller boards.
There is no heating or cooling until communication is restored. Leaving the system idle with E6 on the screen does no harm; trying to force it to run does no good.
Power off, power on: Mitsubishi's own re-check step
Mitsubishi's first instruction for this code is to turn the power off and on again and see whether the code comes back. A homeowner can do that at the breaker.
- Switch the system off at the controller.
- Turn off the breaker that feeds the heat pump. If the indoor unit has a breaker of its own, turn that off too.
- Wait a few minutes before restoring power.
- Turn the breaker on and start the system in its normal mode.
- If E6 comes back, either right away or days later, write down when and call. A code that returns is the result the re-check is designed to find.
Three causes of E6, ranked
- An incorrect or loose indoor/outdoor connecting wire. A conductor on the wrong terminal, or a terminal screw that has backed off, interrupts the signal.
- Electrical noise on the connecting wire or the power supply, enough to corrupt the signal.
- A controller-board fault. This is diagnosed last, after the wire has been proven good.
The history helps sort these. An E6 on the first power-up of a new system points toward a connecting wire that was landed incorrectly, which is the installer's to correct. An E6 that comes and goes on an older system points toward a loose termination or noise.
How urgent E6 is depends on whether the heat pump is your only heat
With a furnace or boiler as backup, E6 is an inconvenience and a weekday appointment is fine. If the heat pump is the only heat source and it is below 0°C outside, treat it as a no-heat emergency and call the 24/7 line at (647) 801-1252.
That second case applies to electrically heated homes and to rural properties beyond the gas mains, in places like rural Caledon, Kilbride and Campbellville, where a cold-climate heat pump has replaced baseboards or an oil furnace. Keep portable heaters clear of anything flammable while you wait, and open the cabinet doors under sinks on outside walls so the pipes stay warm.
How the connecting wire and controller boards are tested
An E6 that returns means the connecting wire and both controller boards need testing. A technician checks each termination, tests the cable conductors for continuity and for shorts, and measures the signal between the units with a meter. Only when the wire is proven good does a board get replaced, because a board is the expensive guess.
Why one mini-split head is not working is the better starting point if only one zone of a multi-zone system has dropped out. A different Mitsubishi code, P8, deals with pipe temperature and not communication.
Before the visit, copy the model numbers from the nameplates on both the indoor and the outdoor unit and note whether the system is single-zone or multi-zone. Board and wiring layouts differ between Mitsubishi lines, and the model numbers tell the technician which service manual applies.
- Diagnosis on site: $90–$150, deducted from the repair if you approve it
- Wiring repairs: priced in writing once the fault is located
- Controller boards: priced in writing by model, with any manufacturer warranty on the part checked first
ZK Mechanical services Mitsubishi heat pumps and ductless systems from Burlington across Oakville, Milton, Hamilton, Mississauga and Caledon. Book heat pump repair, see how we handle Mitsubishi heat pump repair, or send the code and model number and we will reply with next steps. The Mitsubishi error code guide covers the other codes.