Mitsubishi Electric fault code E6
Indoor / outdoor comms failure.
What E6 actually means
The indoor and outdoor units have stopped talking to each other. Nothing is necessarily broken inside either one; the message between them is not getting through.
What causes it
- A loose terminal on the S1-S2-S3 comms wire, usually at the outdoor unit
- Damaged interconnect cable, sometimes rodents, sometimes a bracket rubbing through it
- Water in the outdoor terminal box
- Indoor or outdoor board fault
- Wrong wiring after someone else's install or repair
Most likely first. That order is from what we actually find on site, not from the manual.
Safe to check yourself
No tools, no covers off, nothing to do with gas, refrigerant or wiring.
- Turn the isolator off and back on, wait two minutes, and try again
- Check the outdoor isolator switch is actually on
What we check on site
So you know what you’re paying for.
- Voltage check across S1-S2 and S2-S3 at both ends
- Continuity and insulation test on the interconnect
- Inspect terminals for water ingress and corrosion
- Board diagnostics if the wiring proves good
Can you keep using it?
The unit will not run properly with E6 up, so the question is moot. It is not dangerous, though; nothing about E6 makes the system unsafe to leave connected.
What the fix usually involves
Most E6s are a terminal, a damaged cable or water in the outdoor box, and they are fixed on the first visit. Board replacements are the minority.
Is it worth fixing?
That depends almost entirely on how old the unit is. Past about ten years on hot water and gas heating, and twelve on aircon, a significant repair usually costs more per remaining year than a replacement does — and it’s the same point at which the VEU rebate is worth the most, because old inefficient appliances are exactly what the scheme pays to remove. If yours is young and the fault is a component, we’ll fix it and say so.
Repair or replace? The 10-year rule, and what the rebate is worth →